Review paper. Coastal net fisheries have been implicated in declines of numerous seabird populations but there are substantial difficulties in establishing cause of a population decline. Synthetic nets have been implicated as a major contributor to the decline of several auk populations.

Habitat Effects

Species and Community Effects

Diving seabirds more vulnerable to entanglement in set nets. Number of birds killed depends on their abundance, diving habits and distribution within the fishery area. Incidental catch of seabirds can be very high around colony sites. Large numbers of shearwaters have been caught in nets. Species of particular importance in European terms known to be caught in nets include: red-throated divers, Leach?s petrel, gannet, shag, Brunnich?s guillemot and razorbill. In Britain Great northern diver, Slavonian grebe, scaup, common scoter, long-tailed duck and guillemot can be added to the list. Threat to wildlife depends on netting effort and wildlife concentrations. There is temporal and spatial variation in these threats which may be reduced by manipulating where and when fishing takes place.

Subtidal rocky habitats characterised by encrusting communities that are resilient to predation and invasion are extremely vulnerable to mussel dredging as these organisms often have poor dispersal mechanisms and slow growth rates. Desertification of such habitats recorded in Italy following intensive and destructive mussel dredging. Reefs extremely vulnerable to fishing as they often represent islands in seas of soft sediments making recolonisation from surrounding areas unlikely. Intertidal and subtidal soft sediment communities are vulnerable to fishing and as they are often close to areas of population density, heavily fished.

Bottom fisheries have resulted in the destruction of Zostera beds and saltmarsh vegetation. Calcareous algal bed of maerl destroyed by 8 passes of a dredge in Scotland. Reef building polychaetes Sabellaria spinulosa, seagrass Zostera marina and oyster beds Ostrea edulis destroyed by trawling. Hydroid and brozoan habitats lost in English Channel.

Zostera marina indirectly impacted by increased turbidity, replaced by deposit feeding polychaetes, community composition shifts such as these may resist the recovery of suspension feeding species. Epifauna often play key roles in influencing the structure and stability of benthic communities, modifying benthic boundary flow which further influences sediment characteristics and so the settlement of larvae. Epifauna may also provide a refuge for juvenile species from predators. Organisms which stabilise the seabed can also mitigate the effects of natural disturbances such as storms. Modification of microbial activity induced by bottom fishing, resuspension of pollutants, increased benthic/pelagic nutrient flux. With repeated trawling the intense disturbance may select for species with the appropriate facultative responses, communities will become dominated by juvenile stages, mobile species and rapid colonists.

Large amounts of discards falling to the seabed cause anoxia in bottom sediments the discards decay using up oxygen, kills scavenging organism attracted by the discards. Decaying discards may also harbour disease and have caused the elimination of a scallop fishery in Australia.

Species and Community Effects

Diving seabirds more vulnerable to entanglement in set nets. Number of birds killed depends on their abundance, diving habits and distribution within the fishery area. Incidental catch of seabirds can be very high around colony sites. Large numbers of shearwaters have been caught in nets. Species of particular importance in European terms known to be caught in nets include: red-throated divers, Leach's petrel, gannet, shag, Brunnich's guillemot and razorbill. In Britain great northern diver, Slavonian grebe, scaup, common scoter, long-tailed duck and guillemot can be added to the list. Threat to wildlife depends on netting effort and wildlife concentrations. There is temporal and spatial variation in these threats which may be reduced by manipulating where and when fishing takes place. Longline: Swordfish fishery North Western Atlantic took several times more shark than swordfish resulting in grey seal population rising from 3000 to 45000. Grey seals Halichoerus grupus acted as a primary host for parasites which then infected cod. Population density may have increased stress in seals causing a population decline. Gill nets implicated in the extinction of several species. Adult survivorship is extremely important for marine mammals and birds as they have slow reproductive capacity and low fecundity therefore they are high vulnerable to even moderately increased mortality. Incidental by-catch of highly mobile predatory marine mammals likely to be higher than less mobile species as they are efficient foragers and are likely to be attracted to nets laden with fish. Approximately 500-1000 harbour porpoise caught annually in Danish waters. Catch rate of harbour porpoise approximately 0.1 individuals/km of net/day probably an underestimate. Porpoise populations substantially reduced by the Pacific tuna purse seine fishery. Ghost fishing by discarded and lost netting may be significant and persistent, impacting not only on non-target species such as birds and marine mammals but also on fisheries themselves.

Complete loss of sessile fauna on rocks and cobbles caused by the action of fishing gear on the seabed. Hydraulic dredging causes complete loss of sessile benthic fauna which are killed by the heat. Otter trawling causes massive amount of by-catch including crab, scallops, starfish. Mortality for some species can range from 10 percent in starfish to 90 percent in Arctica islandica after a single trawl this may increase drastically with increased trawling intensity.

Paper reviews primary literature that has estimated collateral and unaccounted fishing mortalities and identifies the key reason behind them. Collateral and unrecorded mortality can include can include: i) avoiding, ii) escaping, iii) dropping out of the gear during fishing, iv) discarding from the vessel, v) ghost fishing of lost gear, vi) habitat destruction (& subsequent), vii) predation, viii) infection from any of the above. The review then aimed to develop a way of reducing the unwanted exploitation and suggest possible mitigation methods.

A broad overview of the effects of gill nets on seabirds including case studies.

Habitat Effects

Species and Community Effects

Worldwide 60 species of seabird reported as being caught in gill nets. In very few cases was it possible to estimate the level of mortality in specific fisheries but net mortality was implicated as a major contributed to large declines in certain populations. Great northern diver and red throated diver thought to be vulnerable. Average number of great northern divers caught per year 15 780 percent of great northern divers caught off Newfoundland entangled in salmon gill nets 20 percent in cod gill nets. Great northern divers caught in nets up to 50m deep.

General principles associated with seabird mortality in gill nets: species at greatest risk are predators which (a) pursue their prey underwater (b) aggregate in dense foraging groups.

Daily catch rates can be very variable. Greatest by-catch occurs during periods when prey occur in areas frequented by fisheries. Magnitude of net mortality for many predators may be a function of prey abundance. Net mortality decreases with distance from colonies of breeding seabirds vulnerable to entanglement. Large kills can be caused by nets set at great depths (ie more than 100m). Net mesh size may be an important consideration in mortality rates.

Location

Fishing Types

Gill nets.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Northridge, S., 1988. Marine Mammals and Fisheries: a study of conflicts with fishing gear in British waters Report to Wildlife Link Seals Group.

Description

Comprehensive resume of recorded by-catches of marine mammals including dolphins, seals, porpoises and otters.

Habitat Effects

Species and Community Effects

Incidental catches of marine mammals by no means rare and are reported in most fisheries in Britain. Data is still too sparse to enable a robust estimate of marine mammal by-catch. Gill net fisheries likely to account for the majority of marine mammal by-catches. 130 grey seals from the Farne Islands and the Orkneys may drown in fishing gear every year. Young animals more vulnerable to fixed nets. Cetaceans and seals only very rarely affected by long-line fisheries, creel, potting or salmon nets. Otters may be significantly affected by creel and eel fyke nets and the latter may have been a significant factor in the decline of otters in East Anglia. Salmon farming may have a significant effect on seal populations locally, estimates in the region of 100 seals caught in anti-predator nets annually with a further 1,000 seals shot by fish-farm operators. The number of seals caught in anti-predator nets, fishing nets in general or shot by fish farm operators does not seem to have had a deleterious effect on seal stocks. Harbour porpoise most vulnerable to incidental catches.

Laboratory based experiment investigating the behaviour of Buccinium undatum exposed to different prey items.

Habitat Effects

Species and Community Effects

Less mobile scavengers such as whelks may take several days to arrive at sites of trawl disturbance. Whelks are well suited to exploit fisheries discards as they are very responsive to chemosensory stimuli exuded from damaged or moribund animals. 98 percent of whelks caught in a beam trawl survive. Whelks are capable of exploiting a wide variety of prey due to their flexible feeding behaviour. In this experiment they ate Liocarcinus depurator, Spatangus purpureus, Trisopterus minutus but not Pleuronectes platessa. Where whelks are common they have an important capacity in utilising energy from dead or damaged animals. Whelks using this competitive advantage may exhibit local population increases and in areas of intense beam trawling, such as the southern North Sea, dead or moribund animals which result from these activities could make up a considerable proportion of the whelk diet.

The Sustainable Harvesting of Ensis (SHARE) project, funded by the European Interreg IIIB Atlantic Area Programme, aims to develop sustainable production strategies for two commercially important species of razor clams that form the basis of a declining wild fishery in parts of the Atlantic coast of Europe. Partners from the UK, Ireland, Portugal, and Spain are collaborating on this three-year project (2004 to 2007), which will take a "seed to market" approach to the production two European razor clam species, Ensis siliqua and Ensis arcuatus. At present, dredging for razor clams is prohibited in the UK (stocks are fished by diver harvesting and collecting on the low intertidal). However, the use of dredges is permitted in Ireland, and a large bed discovered off the east coast in 1997 has been heavily fished since then using hydraulic dredges. There is currently little or no management of this fishery and concerns exist regarding the environmental impact of heavy fishing activity that may result in the classic boom/bust scenario. The Centre for Marine Resources and Mariculture at Queen's University Belfast will work closely with Centro de Investigacions Marinas (CIMA) in NW Spain to develop hatchery, nursery, and restocking techniques for E. arcuatus and E. siliqua. Although it is unlikely in the long-term that stock enhancement will provide an economically viable basis for a sustainable fishery, it will be necessary to develop technical measures to achieve restoration of the depleted stocks in the short-term until a suitable fisheries management regime is in place.

Record of causes of death in 422 cetaceans of 12 species stranded on the coasts of England and Wales between August 1990 and September 1995 via post-mortem examination.

Habitat Effects

Species and Community Effects

Most frequent cause of death in harbour porpoises and common dolphins was entanglement in fishing gear. 38 percent of harbour porpoises and 80 percent of common dolphins diagnosed as being by-caught. The proportion of by-caught harbour porpoises increased from 1990 to 1995. Factors such as changes in fishing effort, technique or location or changes in the abundance or distribution of harbour may account for this. Probably an underestimate of the true incidence of by-catch in cetaceans. Estimates of the number of by-caught harbour porpoises cited as being between 328 and 552 by English fishing fleets on the Celtic shelf. The proportion of starved neonatal harbour porpoises higher than starved common dolphins may relate to the more coastal distribution of harbour porpoises. More coastal distribution of harbour porpoises may also increase their contact with co-factors such as pollutants making them more likely to die from species-specific pathogens than common dolphins. By-catch is a threat to both harbour porpoises and common dolphins around the coast of England and Wales. Of 7 Tursiops truncatus studied only one was determined as being by-caught.

Undersized indivivuals and some non-target species were sieved out and dropped in situ, reducing the potential impact of being removed entirely from the sediment and being dispersed elsewhere. More damage to captured individuals was recorded than standard gear. Gear was selective for associated fauna, resulting in high weight of bycatch in experimental gear.

Location

Fishing Types

Suction dredge.

Reviewed by

Sewell et al 2007

Full Reference

Kennedy, R.J. & Roberts, D., 1999. A Survey of the current status of the flat oyster Ostrea edulis in Strangford Lough, Northern Ireland, with a view to the restoration of it oyster beds. Biology and Environment: Proceedings of the Royal Irish Academy, 99, 79 - 88

Description

A study to assess the potential for restoring native oyster beds in Strangford Lough.

Sampled an off-bottom long-line culture area, a dredged ground culture area, a handpicked ground culture area, and an area without aquaculture. measured the standing biomass, percent cover and growth rate of eelgrass, as well as the density of vegetative and flowering shoots.

Habitat Effects

Species and Community Effects

Found the largest growth rates in areas with off-bottom culture and those without aquaculture; these areas also had the greatest eelgrass biomass, density, and percent cover. Eelgrass growth and biomass were lower in handpicked and dredged culture areas and didnot significantly differ from one another. There were significant site and culture type interactions for most variables suggesting that site-specific conditions may be as influential as culture technique in determining eelgrass growth.

Bottom deposits settled about 20 mins after hauling. Short teeth of these dredges dug in up to Â½ to Â¾ of their length and generated a large mound of sediment in front of the toothed bar. Most was deposited around the sides of the dredge and at times completely filled the dredge opening, particularly when large stones or shells blocked some of the gaps between the teeth. Dredge tracks were distinct, ridges of sediment being deposited each side, but path of the spring-loaded dredge less obvious than standard dredge.

Species and Community Effects

The dredges caused some damage to benthic organisms. Most hauls had a few crabs Cancer pagarus, and starfish eg Marthasterias glacialis broken up by the gear. The teeth also dug out several sub-surface animals including heart urchins Spatangus purpureus and the mollusc Laevicardium crassum. These and other organisms raked up by the teeth appeared to attract several fish and invertebrate predators including juvenile cod adult plaice and dogfish, whelks and hermit crabs.

Authors tried to reduce the organic load from the cultured oysters deposited on the seafloor by hanging an artificial midlayer seafloor that traps some of the sinking organic particles before they reach the seafloor. The artificial seafloors are suspended in the midlayer so that the organic matter decomposes in the aerobic condition. Three kinds of artificial seafloor which were made of oyster shell, bamboo, charcoal, and particle-filtering mat were tested. Monitoring for 69 days showed that the number of benthic animals increased and decomposed organic matter along with bacterial decomposition on the artificial seafloors.

Habitat Effects

Budget analyses of organic matter revealed that the artificial midlayer seafloor made of oyster shell was most efficient to reduce the organic load from the cultured oysters above, showing the highest decomposition rate of 6.6% in 69 days. Authors suggest that the devices proposed in this study will support the future sustainability of oyster culture by accelerating the natural self-purification ability.

Review of interactions between the harbour porpoise and the gill net fishery

Habitat Effects

Species and Community Effects

Harbour porpoise are one of the more vulnerable marine mammals to incidental capture by commercial fishing gear and are particularly prone to entanglement. Nearshore habitats, small size and diet of commercially harvested fish contribute to the magnitude of the incidental and/or directed takes occurring through most of their range.

Incidental capture of cetaceans in gill nets is geographically widespread and considered a severe problem. Most capture dolphins and porpoises although large cetaceans are also vulnerable to entanglement. Large incidental catches can occur in coastal gill net fisheries which can have a greater impact than oceanic fisheries because coastal cetaceans often have more restricted distributions than oceanic relatives. Several proposals to reduce impact are discussed.

Seabird populations have been affected by fishing activities as a result of incidental mortality where birds have become caught in nets and drowned and direct mortality as a result of changes in food supply due to depleted stocks or discards. The paper highlight fisheries management issues that are likely to affect the future conservation status of vulnerable seabird populations.

Habitat Effects

Species and Community Effects

The by-catch of seabirds in longline fisheries is considered to be the most serious fishery issue at present, affecting both pelagic and demersal fisheries. Mitigation measures are legally required in a number of regions and fisheries, but not all fisheries adopt them. Set-nets have also caused a decline in regional seabird populations due to a high mortality rate in monofilament nets. Mortality in the North Pacific of 500,000 seabirds each year until 1992 (fishery was closed) resulted from the high seas salmon (gill net) and squid (drift net) fisheries.

The paper looked at the current research that addressed the question of by-catch. How many species are being caught and removed from the population and what effects this removal has. There were concerns with data limitation as by-catch was often unrecorded when reported or wasn’t reported at all, which led to a level of uncertainty. It was also important to consider the effects of fisheries by-catch on a global scale.

Habitat Effects

Species and Community Effects

Studies showed that a number of marine megafauna are at risk from extinction as a result of fisheries by-catch. Longline fisheries have been linked to decreases in the albatross populations, trawl fisheries have been linked to a the number of sea turtles that wash up dead on the shores and gillnet, driftnet, purse seine and trawl fisheries have led to threats to the populations of small cetaceans. In order to reduce seabird by-catch in pelagic longline fisheries gear modifications have been introduced including bird scaring lines which keep the birds away from the baited hooks, weighted lines which enable the hooks to sink faster out of reach of the birds, side-setting which halves the scavenging area and line-setting devices which place the baited hooks immediately underwater.

sole plate of 4m trawl exerted a force of about 2N/cm2 at commercial trawling speeds. Trawl marks on coarse sand visible up to 52hrs after fishing.

Species and Community Effects

Range of mortalities of discarded, non-target species due to capture and handling. High mortalities for undersized fish discarded, 50 percent or less for most crabs and molluscs and very little mortality (<10 percent) for starfish. Overall decrease of 0-85 percent from initial numbers for different mollusc species (solid-shelled or very small species such as Chamelea gallina, Corbula gibba, Dosinia lupinus and Apporhais pespelicani not affected. More vulnerable species such as Abra alba, Mactra corallina, Ensis ensus, Arctica islandica and Turritella communities had mortalities between 12-85 percent), 4-80 percent for crustaceans Corystes cassivelaunus and Ebalia spp. approx. 30 percent, Eupagurus bernardus showed size dependent mortality 15 percent for large animals and 74 percent for small animals; Callinassa spp. lived too deeply to be disturbed by beam trawling, 0-60 percent for annelids and 0-45 percent for echinoderms A. rubens, A. irregularis, A. filiformis and O. texturata little affected and E. cordatum too deeply buried to be harmed. Considering the high mortality of certain species and the fishing intensity, it can be expected that commercial beam trawling affects the structure and composition of the benthic community in the North Sea. Benthic animals damaged, dislodged or discarded by beam trawls may contribute significantly to the diet of scavengers whose populations may thus become enhanced. Investigations into scavengers showed that dab, gurnard, dogfish and whiting increased intake of prey after fishing. Dab fed largely on bivalves Arctica, Acanthocardium, Donax and Spisula and crustaceans Upogebia and Callianassa the latter of which are not normally accessible to them. Gurnards and whiting fed on dislodged amphipods and whiting fed on the damaged burrowing heart urchin Spatangus purpurreus. Fish rapidly migrated into trawled areas to feed on animals damaged or disturbed by fishing.

Paper discussing whether changes in behaviour caused by human presence are likely to be good measures of the relative susceptibility of species. The authors suggest that their use may result in confusion when determining conservation priorities.

Paper examining whether oystercatchers are able to compensate for lost feeding time, by increasing their feeding rate. Study used five tagged birds feeding on mussel beds and monitored responses to disturbance.

Habitat Effects

Species and Community Effects

The birds did not increase their rate of fedding, but fed for longer periods, indicating some 'slack' in their normal feeding time.

Paper examining the response of birds, in estuaries in response to shellfishermen.

Habitat Effects

Species and Community Effects

The dunlin Calidris alpina was driven to alternative feeding grounds which were less profitable with lower prey densities, and a higher density of conspecifics. Whilst the latter is likely to reduce the feeding efficiency of an individual bird, the risk of predation could be decreased due to the increased group density.

Location

Fishing Types

Competition for resources (general).

Reviewed by

Sewell et al 2007

Full Reference

de Groot, S.J., 1995. On the penetration of the beam trawl into the sea bed ICES . C.M. 1995/B:36

Description

Review of data on penetration of depth of ticklers and chain arrays of beam trawls.

Habitat Effects

Under normal working conditions beam trawls influence only the top layers of the sea bed up to 30mm on muddy ground and up to 10mm on sandy ground. Summary of results to date suggest average penetration depth 4-7cm. The depth depends on the bottom type and structure of the ticklers and does not always penetrate as the gear moves over the seabed at speeds of 6-7 knots.

Study describes a method of quantifying the effect of disturbance, based on measuring the trade-off between resource use and risk of disturbance. Study examines the impact of disturbance on Pink-footed geese, Anser brachyrhynchus feeding in arable fields. The impact of disturbance is calculated based on the food remaining and the number of geese this food would have been able to support.

Chapter examines the seabird mortality in gill nets in the North Pacific.

Habitat Effects

Species and Community Effects

Some of the worst mortality rates are associated with the squid and salmon drift-net fisheries in the North Pacific, which are estimated to have killed around 500,000 seabirds per year before its closure in 1992

Report describes an on-board observer study of the seabird bycatch taken by Norwegian offshore longline fishing vessels in the Norwegian Sea in 1997 and 1998. Report details the significant level of seabird by-catch in the industry and mitigation methods are suggested.

Reviews current knowledge of environmental modification or conflicts with other species at seed collection, seed nursery and on-growing, and harvesting stages of the cultivation process.

Habitat Effects

Species and Community Effects

Seed collection - subtidal dredging for seed mussels likely to be confined to relatively small areas of seabed because they occur in dense aggregations in discrete areas. UK licensed areas from unstable beds which are likely to be lost anyway. Non-target species probably adapted to large-scale natural disturbance so likely to recolonise rapidly but in extensive heavily exploited fisheries, such as the Wadden Sea, the entire mussel stock was removed in 1990/1 resulting in increased mortalities for eider duck and reduced breeding success for oyster catchers. May be some effects associated with intertidal collection (trampling, disturbance of foraging birds and removal of winter food source). Few impacts likely from spat collectors, continuous relaying of cultch leads to habitat modification which may increase diversity. There are also risks associated with the introduction of alien species.

Ongrowing- Effect depends on habitat, type and scale of cultivation. Introduced structures effect local hydrography and provide a settlement surface, high densities increases local oxygen demand and elevates input of organic matter however beds used to be extensive and they fulfil an important role in the retention of phosphorus and nitrogen. May be eutrophication beneath mussel lines if not enough tidal flow to disperse particulate matter. Decreases in abundance of macrofauna and increases in meiofauna beneath oyster trestles been measured. In the USA insecticide is sprayed on intertidal areas and ground may be harrowed prior to cultivation. Addition of gravel or shell, formation of mussel mud and use of protective netting induces localised changes in benthic community composition. Small-scale culture seems to have only very limited effects on local benthic communities. Cultivation sites may conflict with bird feeding or roosting sites but probably only problematic if cultivation areas cover significant part of the feeding grounds.

Harvesting- restriction harvesting to early winter could ameliorate site restoration if main mechanism for recolonisation is by larval settlement. Suction dredging or mechanical raking affects the habitats. Recolonisation rates likely to differ between habitat types.

Study into the influence of fisheries on interspecific competition in sympatric populations of hermit crabs.

Habitat Effects

Species and Community Effects

Starfish and decapod Crustacea are among the most important megaepibenthic scavengers that aggregate in areas of fishing activity but recent work indicates that scavengers are far more selective than presumed previously. They avoid carrion that is phylogenetically similar and may avoid carrion that attracts potential predators. The authors suggest that additional food resources arising from fishing activities are distributed unequally between sympatric populations of hermit crabs as a consequence of differences in their competitive abilities. This may provide a mechanism whereby fishing activities could lead to changes in the structure of crustacean scavenger populations.

Study examining the effect of mixed species flocks on the roseate tern. Authors examined foraging behaviour in birds of single species and mixed species flocks.

Habitat Effects

Species and Community Effects

The authors discovered that the terns were able to forage more effectively in single species flocks, but could not detect any negative impact on suvivrship or reprodcutive success.

Location

Fishing Types

Competition for resources (general).

Reviewed by

Sewell et al 2007

Full Reference

Auster, P.J., 1998. A conceptual model of the impacts of fishing gear on the integrity of fish habitats. Conservation Biology, 12, 1198-1203

Description

Author develops a conceptual model of gear impacts across gradients of habitat complexity and levels of fishing effort. Habitats are grouped into 8 general categories and scored according to their complexity.

Habitat Effects

The conceptual model shows the response of the range of seafloor habitat types to increases in fishing effort scored from 0 to 4. It shows a range of changes in habitat complexity based on the effects of fishing grear and predicts reductions in the complexity provided by bedforms from direct smoothing of gear.

Authors suggest that effects of bottom trawling are the marine equivalent of forest clearcutting, acting as a major threat to biological diversity and economic sustainability, and occurring at a rate two orders of magnitude higher than forest loss worldwide. Reasons include reduction in structural complexity of benthic communities, alternation of biogeochemical cycles, and slow recovery after disturbance. The effects can be large and long-lasting on benthic communities as well as young stages of some commercially important fishes although other species benefit when structural complexity is reduced. Recent experimental studies on trawling and dredging impacts on benthic communities are tabulated. The paper describes the extent and severity of the activity noting that advances in fishing technology have virtually eliminated de facto refuges from trawling, and that frequency of trawling is orders of magnitude higher than other severe seabed disturbances. It calls for the establishment of refuges free of mobile fishing gear, modification of fishing methods and a precautionary approach to management.

Effects on habitats and benthic communities most readily identified and last longest in those areas that experience infrequent natural disturbance. Initial effects can be dramatic, additional effects more difficult to detect. Authors concluded that once an ecosystem enters the fished state, diversity, structure and fish production tend to remain relatively stable across a wide range of fishing intensities. Fishing has accelerated and magnified natural declines in abundance of many forage fishes and this has led to reduced reproductive success and abundance in birds and marine mammals. Dramatic and apparently compensatory shifts in the biomass of different species in many fished ecosystems are considered to often be driven by environmental change rather than indirect effects of fishing. When predator or prey fill a key role, fishing can have dramatic indirect effects on community structure

Species and Community Effects

Location

Fishing Types

Various (Not listed).

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Lindeboom, H.J & de Groot, S.J., 1998. The effects of different types of fisheries on the North Sea and Irish Sea benthic ecosystems . RIVO-DLO Report C003/98

Description

Report on the results of international research project investigating the effects of different types of fisheries on the North Sea and Irish Sea benthic ecosystem. Provides an overview of the effects of bottom trawling on marine communities with chapters on physical impact, direct mortality due to trawling, scavenger response to trawling, comparison of undisturbed and disturbed areas and long term trends in demersal fish and benthic invertebrates.

Review of environmental issues associated with different types of aquaculture conducted around the world. Describes different systems of aquaculture then covers environmental impact of the facilities (e.g.. mussel cages and floating cage farming), and of the use of chemicals including antibiotics. Sections on waste minimisation, wastewater treatment systems and environmental management systems for aquaculture.

Comparison of historic (1946-1951) and recent data on benthos in locations some of which have been subject to heavy scallop dredging over the intervening years, some to little dredging.

Habitat Effects

Species and Community Effects

Changes apparent regardless of intensity of dredging. In heavily dredged areas there was extreme physical disturbance, increased polychaete:mollusc ratio, loss of some fragile species and an increase in the predominance of scavenger/predator species. Changes in lightly dredged areas included loss of a number of species including some potentially fragile tube-dwellers. Reasons for these changes not apparent.

Simulation in test tank of effects of otter trawl door on infaunal bivalves when moving across a relatively dense, level, sandy seabed. Six species of bivalve were placed in the test bed in typical life positions.

Habitat Effects

A mound of sediment in front of the door formed a single rounded berm with adjacent shallow U-shaped depression which represented the scour furrow.

Species and Community Effects

All bivalves within the scour path at the sediment/water interface were displaced but only 5 percent sustained major damage. Shallow burrowing bivalves in the scour path were redistributed and concentrated along the berm. Exposure on the seabed would make them vulnerable to predation. Increased sediment stress was recorded to depths occupied by deep burrowers but in this experiment the transient elevated stress levels were considered to be of insufficient magnitude to cause shell damage. Possible behavioural or physiological effects on the bivalves unknown.

Review of fishing effects on habitat. Common themes to emerge included immediate effects on species composition and diversity and reduction in habitat complexity. Recovery variable depending on habitat type, life history of component species and natural disturbance regime.

Habitat Effects

Species and Community Effects

Location

Fishing Types

Various (Not listed).

Reviewed by

Gubbay and Knapman, 1999

Full Reference

ICES 1998. Report of the working group on environmental interactions of mariculture ICES . CM 1998/F:2. Ref:ACFM+ACME+E

Description

Papers from working group meeting. Sections on following strategies in coastal cage farming and associate research needs, minimum separation distances between cage farming sites, on coastal management and mariculture and on escapes.

Experimental study to investigate changes in benthic communities and sediment composition associated with clam cultivation. Trials with four treatments, clams with net covers, net covers only, control plots without clams or net covers and control plats without clams, net covers or human activity. Sediment of the trial area was a stable muddy sand.

Habitat Effects

With net cover, netting and the green alga growing on it caused an increase in sedimentation rate, and slight increase in proporation of silt.
Harvesting by hand raking, followed by suction dredge. Suction dredge increased sediment load in the water which dispersed to near background levels within 40m of the device. A trench about 10cm deep was left by the harvester which took about 3-4 months to fill.

Species and Community Effects

With net cover, number of worm species increased substantially beneath netted plots irrespective of whether clams were present. Increase occurred within 6 months of placement and still present 2.5 years after seeding when clams were harvested.

Harvesting by hand raking, followed by suction dredge. Hand raking caused a reduction of 50 percent in abundance and diversity of species and suction dredging, a reduction of 80-90 percent. Regeneration of species diversity and abundance, after harvesting in the winter was completed by the following summer.

By-catch of Phocoena phocoena occurs in all areas where bottom-set gill nets and the harbour porpoise co-exist. Levels of bycatch from this industry may even be underestimated as individuals often fall out of nets during hauling and may not be counted in surveys. Some harbour porpoise by-catch also occurs in drift net fisheries. Other small cetacean species are captured in pelagic trawls, drift nets, and bottom-set nets.

Relevant section of the report describes a study to identify the recoverability of Arenicola marina populations following depopulation by bait digging anglers.

Habitat Effects

Species and Community Effects

Over a six-month experimental period there was no significant increase in the density of worms in depopulated areas. Control populations remained approximately constant during June and July but decreased in density throughout the remainder of the study, leading to some convergence of treatment and control areas by the end of the experiment.

The report includes description of recovery of Ascophyllum nodosum following hand gathering.

Habitat Effects

Species and Community Effects

If stumps of harvested Ascophyllum are left at 10 - 20 cm, re-sprouting will occur and the plant will be harvestable after 3 - 6 years. If the whole plant is taken, recovery is slow due to slow recolonisation.

A review of the geographical distribution of the Sabellaria alveolata including looking at the effects of trampling on Sabellaria alveolata reefs. This trampling might be associated with angling, bait collection and gathering intertidal organisms.

Habitat Effects

Species and Community Effects

Following the damaging effects of trampling, worms are often unaffected and may be able to rebuild their tubes rapidly.

Location

Fishing Types

Angling. Hand gathering. Bait collecting.

Reviewed by

Sewell & Hiscock 2005

Full Reference

Rogers, A.D., 1999. The biology of Lophelia pertusa (Linnaeus, 1758) and other deep-water reef-forming corals and impacts from human activities. International Review of Hydrobiology, 84, 315-406

Description

Review of the biology of deep water coral reefs, particularly Lophelia pertusa, including an extensive review of impacts from human activities. One section describes the potential and know impacts of deep-sea fishing.

Habitat Effects

Physical damage caused by trawling will damage the three-dimensional structure of the reef resulting in reduced habitat complexity.

Species and Community Effects

Deep sea fishing is considered to be one of the main threats to Lophelia reefs in the North-east Atlantic and is known to have had significant impacts on reefs in other parts of the world. Many of the deep-sea fish species targeted by fishermen in these areas are also particularly sensitive to overfishing due to their long life-history characteristics. Disturbance to the reef may result in an'alternative low diversity 'disturbance community', particularly following high intensity trawling. Corals are also likely to be damaged by the settlement of resuspended sediments.

Study looks at the effects of different fishing gears, indicating which components of the gear are likely to cause the most severe affects and also how these gears affect community structure.

Habitat Effects

Observations by divers and ROV have shown that trenches have formed after the passage of dredges over the substrate, with visible sediment rings being deposited on each side of the track. Effects of dredging can include: bringing stones to the surface after repeated dredging, damage to reefs and chemical changes & sediment compaction. The effects of hydraulic suction dredging can also be seen immediately with visible trenches being left in the seabed. Although these trenches started to fill after 5 days and were not visible after 11 weeks the sediment in the tracks that had been fished remained fluidized for a much longer period of time.

Species and Community Effects

Effects on non-target species can range from none to displacement or injury including mortality depending on vulnerability of the organisms. Mobile epifauna may be capable of avoid capture depending upon the gear type; where as sessile organisms may be more vulnerable. Some species that provide a habitat for other organisms may be caught and removed.
A result of hydraulic suction dredging is that non-target species can become distributed much further away from the dredging location.

Study aimed to provide a framework to evaluate the closure of scallop fisheries around the British Isles on a rotational, seasonal and permanent basis, aiming to reduce the impacts that commercial scallop dredging has on the scallop stock and benthic communities. A review of available information on rotational, seasonal and permanent closures was also conducted.

Habitat Effects

Seasonal fishery closures: benefit to long-lived fragile benthos is likely to be limited.
Rational fishery closures: sandy seabeds can tolerate a low level of disturbance as they are naturally dynamic environments. Short rotational closures (<5 years) should help enhance diversity of these communities.
Permanent closures: as a result areas of a larger habitat may be preserved, this could allow for the re-establishment of broad communities. Biogenic habitats are very vulnerable to disturbance as they are slow-growing, long-lived and recruitment is regular (e.g. horse mussels), as a result of this protection is probably best achieved through permanent closure.

Species and Community Effects

Seasonal fishery closures: benefit to long-lived fragile benthos is likely to be limited; some fast growing species (erect bryozoans and hydroids) may take advantage of the reduced disturbance. Organisms that have similar life-histories to scallop are the ones that are likely to benefit the most from a seasonal reduction in disturbance.
Rational fishery closures: short rotational closures (<5 years) may increase the abundance and size of vulnerable species and habitat-forming organisms. However, once dredging resumes these recovered communities can quickly return to their dredged state. To allow for these organisms to recover longer rotational closures (5-10 years) should be considered.
Permanent closures: this type of closure can affect the target species though larval export and spill-over of adults into adjacent grounds, which may increase the sustainability of the fishery. Genetic diversity may also be maintained through a reduction of fishing pressure by preventing the stocks from being dominated by slow-growing genotypes. This type of closure may also be the only way to protect key species that are long-lived or fragile.

The study was conducted to clarify the biology of Ensis arcuatus and to provide evidence as to how the population might be exploited in a sustainable way. Samples were collected by diver surveys to quantify the razor clam population. At each station a quadrat was placed on the seabed and 1 litre of granular salt was poured over the substratum, after 10 minutes any clams that appeared were collected by hand. Samples were also collected from the commercial fishery. All harvested clams (diver and commercial collection) were taken back to the laboratory where they were measured and weighed. Around 30 clams were split lengthways and a section of the gonadal tissue was extracted, this was used to assess gonad development.

Habitat Effects

Large concentrations of Ensis siliqua were associated with fine sand, whereas Ensis arcuatus occurred in algal gravel (including maerl) and shell sand. The highest densities of Ensis arcuatus occurred on a sand substratum with concentrations of Zostera.

Species and Community Effects

Location

Fishing Types

Hydraulic dredge.

Reviewed by

Sewell et al 2007

Full Reference

Eno, N.C., MacDonald, D.S. & Amos S.C., 1996. A study on the effects of fish (crustacea/mollusc) traps on benthic habitats and species Report to the European Commission..

Description

Experimental study on the effects of Nephrops creels and lobster and crab pots on benthic habitats and communities in a number of locations/habitats. Quantitative effects of one month's fishing using crab and lobster pots. Locations:

Habitat Effects

Species and Community Effects

Sites in Scotland - Descending creels build up a small pressure wave which caused the sea pens Pennatula phosphorea, Virgularia mirabilis and Funiculina quadrangularis to bend before the creel made contact. This removed the tip of the sea pen from damage through impact. After smothering or uprooting all three species reinserted and uprighted themselves when in contact with muddy substrate. No lasting effects on muddy substrates.

Devon/Wales - Rocky substrate habitats and communities at a depth no deeper than 23m below chart datum subjected to lobster and crab potting relatively unaffected by fishing activity. Experimental and control plots 30mx12m in Devon and 50mx20m in Wales. Pentapora foliacea found broken after hauling although unclear whether this was due to fishing. Eunicella verucosa bend under the weight of pots and then return to an upright position afterwards. Slow growing and long lived Eunicella verucosa previously considered highly vulnerable to damage. One month's active fishing using crab and lobster pots caused no difference in abundance of species between control and experimental study plots. Abundance of some species increased after potting in comparison with their abundance before potting. Potting did not have a detrimental effect on the abundance of species studied.

Experimental simulation of 12 lost parlour pots revealed that they may actively fish for up to 270 days and remain baited for between 8 and 27 days. Catch rates highest during first month. Brown crab catches showed slight temporary decrease after bait depleted and subsequently fairly constant. Spider crab catch declined steadily. In time condition of the catch deteriorate, wrasse showed skin damage and limb loss increased markedly the longer crustaceans remained in the pot. Incidental observations in the vicinity of the pots shows several had moved over and broken Pentapora colonies. Pots moved down the gently sloping seabed until constrained by mainline tightening.

The paper reports on two studies. The first examined the effects of an acoustic pinger bearing, bottom set gill net on the movements of harbour porpoise. The second examines the effects of the same type of pinger on herring in the Baltic Sea. The latter study was to explore the hypothesis that pingers deter the harbour porpoise indirectly as a result of prey redistribution.

Habitat Effects

Species and Community Effects

In control studies, harbour porpoise were not affected by the presence of bottom set gill nets. Pinger operation resulted in an exclusion zone around the net ranging from 130 m to 1140 m. Study 2, indicated that herring catches increased with the presence of pingers, indicating that porpoise distribution could not be associated with prey redisitribution.

The study examined the effects of a 3m wide, 120 kg box dredge with 5 - 7 cm long teeth and a net bag, on the benthos of an offshore, sandy, seabed community. The study also included a comparison between a control (unfished) ground and a fishing ground.

Habitat Effects

The upper 6 cm of sediment was disturbed and 50 percent of epifaunal organisms were removed along a flattened track with small heaps of sediment running along each side.

Species and Community Effects

Experimental trawling induced a modification in the macrobenthic community, that was most evident immediately after the trawl. This included the removal of epifauna and an increase in mobile scavenging species. The authors suggest that recorded changes to the meiobenthic community were probably due to sediment disturbance. These changes were recorded after one week. Comparisons between the control grounds and fishing grounds showed that fishing grounds had significantly fewer species and number of individuals and significantly lower biomass of macrofauna, indicating significant long-term effects of fishing.

Study of benthic species composition in a mussel bed following experimental dredging in a brackish sound.

Habitat Effects

A 2 - 5 cm deep furrow was created in the sediment by the dredge.

Species and Community Effects

Fifty percent of mussels were directly removed by the dredge in both dredged areas.Immediately after dredging and for 40 days after dredging, a significantly lower number of species were recorded from the dredged area compared to control areas. Biomass accumulation of mussels in the dredged area was significantly lower, indicating that the disturbance to the mussel bed caused by the dredge reduced the growth rate of mussels.

Examining impacts of Hydraulic dredging, Chamelea gallina fishery. Study took place in area of fine, well sorted sands.
The commercial hydraulic dredge used comprises a 2.4-3 m wide rectangular cage weighing 0.6-0.8 tonnes, mounted on two sledge runners to prevent it
from digging into the substratum to a depth of more than 4-6 cm.
Single tow. Sampling 4 times before (14th June-7th July) and 4 times after (11th -25th July) dredging

Habitat Effects

Species and Community Effects

Measured 1 day peak in scavenging species. When all species groups were studied together, no discernable impacts from HSD were recorded. Where Bivalves, polychates and deposit feeders were studied separately, a discernable affect was noted.

Experimental beam trawling trials to investigate effects on megafauna immediately after fishing and 6 months later on two seabed types - mobile megaripple structures and stable uniform sediment. Control and fished areas were sampled.

Habitat Effects

Species and Community Effects

Short term changes (within ca. 24hrs) were recorded in the megafaunal community in stable sediments but not in the mobile sediments. There were decreases in the relatively slow moving megafauna eg Aphrodita aculeata, Macropodia deflexa and Asterias rubens. Some mobile species (eg. Pagurus bernhardus and Ophiura ophiura increased in the trawled area and are known to migrate into areas of fishing disturbance. There were also increases in some relatively sessile species eg. Mya truncata in the trawled areas but not statistically significant. The effects on the megafaunal community were not uniform, even though the fished areas were completely swept by the gear at least once. Six months later, seasonal changes had occurred in both communities and the effects of the trawling disturbance were no longer evident.

No significant change in biomass of hydroids and Alcyonium digitatum recorded immediately after fishing although these organisms were the largest proportion of the biomass of beam trawl catches at the study site. Repeated and more intense trawling effort is likely to have a greater effect on these organisms.

Three year study into impact and recovery of habitat and marine benthic communities from suction and tractor dredging to harvest cockles.

Habitat Effects

Species and Community Effects

Suction dredging had a statistically significant effect on infauna leading to up to a 30 percent reduction in number of species and 50 percent reduction in number of individuals. These effects were not seen with tractor dredging

Field experiments were carried out to assess the impact that mechanical clam harvesting can have on a large scale, with aim of providing data to resource mangers and increase the scope of benthic ecology. To see what effect this physical disturbance has four things were considered: i) recruitment success, ii) biomass of seagrasses, iii) density of bay scallops and iv) density of other benthic macrofauna. Two sites within an estuarine habitat were selected, seagrasses and sand flats with 6 plots in each site. Each plot was sampled before harvesting to estimate the abundance of bay scallops, hard clams and seagrasses. The harvesting was applied on two occasions and then sampled on 5 subsequent occasions to test for any effects. Of the 6 plots 2 were left untouched in each site, 1 was as a control and the other was subjected to low intensity harvesting.

Habitat Effects

As a result of raking and light intensity clam-kicking the biomass of seagrass beds was reduced straight away by ~25% of the control levels and a full recovery of the site occurred within a year. As a result of high intensity clam-kicking the biomass of seagrass beds was reduced straight away by ~65% of the control levels. Recovery didn’t begin until 2 years later and even 4 years later the biomass was still ~35% lower than the control levels.

Species and Community Effects

Effect on target species: the recruitment of clams varied between the two habitat types. Although there was no clear difference in recruitment on seagrass beds as a result of different intensities of harvesting, there was a difference on the sand flats. The recruitment was lower on the plots that were intensively harvested when compared to the control plot.
Effect on other species: the density of the bay scallop decreased as the seagrass beds decreased, this occurred across all intensities of harvesting. Clam harvesting did not affect the species composition or density of small benthic macroinvertebrates across any of the harvesting intensities.

Study showing how individual-based behavioural models can establish critical thresholds for the frequency with which wading birds can be disturbed before they die of starvation.

Habitat Effects

Species and Community Effects

Modelling shows that the birds can be disturbed up to 1.0-1.5 times/h before their fitness is reduced in winters with good feeding conditions (abundant cockles Cerastoderma edule and mild weather) but only up to 0.2-0.5 times/h when feeding conditions are poor (scarce cockles and severe winter weather). Individual-based behavioural models enable critical disturbance thresholds to be established for the first time.

Study to examine the long and short term effects of clam fishing with a hydraulic dredge on a deep (70 - 80m) sand bank over a period of three years. The seabed was low relief, with burrows, pits and polychaete tubes.

Habitat Effects

The most obvious effect of dredging was a dramatic change in seabed topography due to the numerous deep (20 cm), wide (4 m) curvilinear furrows that were cut by the dredges. The loss of burrows, tubes, and shells through destruction or burial, and local sedimentation created a smooth surface. After one year, furrows were no longer visible on video, but still showed up using sidescan sonograms.

Species and Community Effects

Densities of large burrows were reduced by up to 90 percent after dredging with no signs of recovery after 3 years due to the high mortalities of their architect, the propeller clam, Cyrtodaria siliqua.

Study examining management of scallop fishery from late 1980s until 2002 onwards.
Various management methods discussed including complete fishery closures and an agreement between 3 fishery regulators to close at least one area to fishing every season to prevent recruitment over fishing . A minimum of 20% of the stock must be above minimum size before fishery is able to open.

Preliminary observations of wintering waders, following the creation of the Banc d'Arguin (Arguin Shoal) Reserve, a sandy oceanic islet.

Habitat Effects

Species and Community Effects

wader population increased from 20,000 to 220,000 individuals, approximately 200,000 of these being dunlin. Dunlin, grey plover and oystercatcher adopt a tidal activity cycle, feeding at low tide and resting at high tide. Curlew, bar-tailed godwit and snipe adopt a nycthemeral activity cycle, feeding preferably at night. It appears that the possibility of recuperating and resting on a site emerged at high tide (preferably an islet) which has absolute tranquility is a primordial necessity for a durable stationing for waders. The role of the Reserve is important due to disturbance caused in the Bay of Arcachon by oyster-culture, hunting, etc.

Survey was carried out to test the following statements: high fishing effort
i) reduces diversity and evenness,
ii) reduces observed maximum body mass,
iii) favours a few body mass classes,
iv) increases the steepness of the slope of number-size spectra,
v) shifts abundance and biomass distributions among species toward those of a disturbed community,
vi) changes species composition.
The survey was conducted in around 100m of water to over physical disturbances, 4 areas were chosen, with 2 stations in each. A bottom trawl was carried out in late May and early June 2001, a 2m beam trawl was used to collect samples of demersal fish and invertebrate megafauna.

Habitat Effects

Species and Community Effects

Species richness was greatest in the moderately exploited areas (B & D) than in the heavily exploited areas (A & C), although this difference was not considered to be significant. Species diversity however showed a significant difference between the areas, diversity was greatest in the moderately exploited areas and than the heavily exploited areas. An abundance-biomass comparison indicated that areas B & D showed an undisturbed pattern with the abundance curve below the biomass curve, for areas A & C the curves crossed indicating a disturbed pattern. The dominant species also varied between areas. Area B was dominated by an opportunistic, commercial species, Nephrops norvegicus, area D was dominated by a sensitive echinoderm Astropecten spp., which was only found in this area. In areas A & C opportunistic carnivores were the dominant species, Liocarcinus depurator in area A and Munida bamffia in area C.

Paper outlines the main findings of work conducted in both the Bay of Brest and the Clyde Sea area to assess the past and future impacts that bivalve fishing can have on maerl beds. A habitat that is highly vulnerable to bivalve dredging.

Habitat Effects

Evidence was presented for the advantages and disadvantages of the exploitation of maerl beds through scallop dredging. However, the conclusion was that the protection of maerl is more advantageous than its destruction as a result of dredging. The reason for this is that scallop dredging on maerl beds reduces the complexity, biodiversity and long-term viability of these very slow growing habitats. Although the deep burrowing organisms which can make up a large proportion of the infaunal maerl biomass can survive dredging in high numbers, these organisms are still vulnerable when juveniles are present at the surface.

Species and Community Effects

The target species can also benefit from maerl bed conservation as these grounds can provide broad-stock areas for bivalves, which can in fact enhance the recruitment of juvenile scallops.

Study examining the effects of scallop dredging for sea urchins and scallops on the proportion of sea urchins damaged during the harvesting operation, the impact on and subsequent recovery time of the associated benthic flora and epifauna, and the impacts on the bottom substrate. Diver surveys were carried out immediately before and immediately after and three and six months after the passage of a scallop dredge. Two sites were chosen, with an experimental and control plot at each site.

Habitat Effects

Boulders of varying sizes were dislodged and overturned by the dredge.

Species and Community Effects

At both experimental sites, a decrease in urchin numbers and an increase in broken urchin tests was observed following the harvesting operation. There were significant changes in numbers of predators. The breakage rate of kelp was also increased as a result of dredging.

Study examining Wild populations of Crassostrea gigas originating from farmed stock in a semi-enclosed bay. Boergneauf Bay had a total of 1,000 ha of oyster beds at the time of study. Authors surveyed colonisation of oysters on variety of substrates in the study area.

Habitat Effects

Species and Community Effects

Authors noted that ‘wild’ oyster populations in the bay were approximately 2.4 times greater than farmed oyster poulations, representing significant trophic competition, potentially inhibiting the growth of farmed oysters.

Assessment was conducted into the effects of scallop dredging on the spatial and temporal trends of non-target species caught as by-catch. By-catch data was collected during the annual scallop stock surveys conducted in April (108-130 samples collected annually).
Total catch from each tow was weighed and a random 25kg subsample was taken, all scallops and by-catch species were counted and weighed and abundance and biomass was estimated.
Catch and fishing data was obtained from log books from 1972-2001, fishing effort was recorded as dredging time (the time elapsed between the first and last haul of each fishing day).

Habitat Effects

Species and Community Effects

The data from the annual stock assessment showed that 42 by-catch species were recorded, 22 taxa were excluded as the dredge sampled them inadequately (19 demersal fish species, 2 pelagic invertebrates and 1 bivalve mollusc). Of the taxa that remained only 8 were present in more than 60% of the tows. The biomass of the 10 most abundant by-catch species accounted for 98.8% of the total biomass by-catch, of this Modiolus modiolus accounted for 32.3% and Cucumaria frondosa accounted for 25.3% of the benthic by-catch.
The macrobenthic community showed similar aspects of disturbed communities elsewhere, diversity and species richness was generally low and the dominant taxa included starfish, crabs, hard-shell gastropods and large bivalves. However, there were no evidence of any major impacts of scallop dredging on the abundance and distribution of by-catch taxa. Scallop dredging started in 1972 and by-catch data is only available from 1993 onwards and data for no dredged areas is limited. For this reason it is likely that scallop dredging had already altered the community structure and removed sensitive species before the by-catch data started.

Location

Breidafjordur, west Iceland

Fishing Types

Scallop dredge.

Reviewed by

Sewell et al 2007

Full Reference

Breen, P.A., 1987. Mortality of Dungeness crabs caught by lost traps in the Fraser River Estuary, British Columbia. North-American Journal of Fisheries Management, 7, 429-435

Description

Experimental study on catches and mortality and 10 simulated lost traps, left in place for 1 year.

Habitat Effects

Species and Community Effects

During this time 169 crabs (Cancer magister) were caught, nearly all males, and about half died. This despite ?escape ports? to allow crabs under the legal minimum to escape. Study revealed that the traps continue to attract crabs long after initial bait has gone, and that catch rates were as high after 1 year as 2 weeks after the start of the study.

Experimental dredging using tractor towed cockle harvester at Burry Inlet (east of Whiteford Point and northern edge of Llanrhidian Marsh).

Habitat Effects

Vehicle tracks and dredging furrows created.

Species and Community Effects

Dredging attracted black-headed and common gulls which fed on very small prey items lying on the surface of harvested furrows including Crangon, Corophium, broken cockles, intact small cockles which pass through the drum, and polychaetes. The number of birds attracted and the places they fed depended on the abundance of prey items revealed by harvesting and presence of people. Peak count at Llanrhidian was 200 black-headed gulls and 55 common gulls, mostly adults which fed preferentially in the most recently harvested furrows. Other species present were curlew, dunlin and oyster catchers.

The increased feeding activity of birds was short lived, 14 days for oystercatchers and 7 days for gulls and small waders. Significant reduction in bird feeding activity apparent thereafter and still detectable after four months. Oystercatchers responded more quickly to changes suggesting harvesting may have been less disruptive or recovery quicker.

Overall the short term increase in the number of gulls and waders in the harvesting area was followed by a long term significant reduction in feeding opportunities for bird species. Birds may then leave to find food elsewhere, leading to the considerable alteration in normal seasonal distribution pattern of shorebird populations. Average density of birds were reduced in this trial by between 15 and 75 percent in harvested area.

In recent years mussels have begun to settle over the cockle beds causing problems for the cockle fishery, resulting in a request for the mussels to be removed. As a result conservation managers are concerned that the mussels may be providing a high-quality food source for oystercatchers and the removal of these beds could cause problems. Behaviour-based model was used to assess oystercatcher feeding on cockles and mussels in Burry Inlet and its predictions tested against their distribution across the beds and the amount of time spent feeding. The possible effect caused by mussel removal in terms of mortality rate and body condition was also explored.

Habitat Effects

Species and Community Effects

Observations indicated that 75% of the birds fed on cockles, 18.75% fed on mussels and 6.25% fed on crumble, the model predicted values within a 95% confidence interval of these observed values. The model also predicted the amount of time spent feeding on cockles and mussels with similar precision, when compared to the observed times (model over predicted by 40 minutes across 4 comparisons). At current bird population sizes it was predicted that the shellfish stock would have to be reduced by 50% (from 2000/01 levels) to cause noticeable mortality or emigration. It was also shown that at current stock levels the removal of mussel crumble would have little to no effect on mortality (increase by less then 0.5%). However, if cockle and mussel stocks were to be reduced (by 25%) the reduction in crumble would be more important, between 5 & 7% of oystercatcher populations were predicted to die.

Investigated the use of tractor towed cockle harvester on invertebrate fauna.

Habitat Effects

Species and Community Effects

Smaller interstitial forms were not greatly affected in most cases significant reduction in species numbers occurred immediately after dredging with continued decline for at least two weeks subsequently. After that a few species showed signs of some recovery others did not, although seasonal trends were obviously important for several of the latter type. Effects at Site A (more tube dwelling and sedentary species) were obvious for longer than 3 months and the dredged area was still visible after 6 months. At Site B (more mobile fauna) natural winter weather disturbances resulted in changes of greater magnitude than those caused by dredging. Results suggested the importance of a stable environment, including surface microflora, for maintaining certain diverse community types and also revealed interesting patterns. Some types of benthic intertidal communities would be adversely affected by commercial tractor towed cockle harvesting.

General conclusions from both this study and a 1990 study at Lavan sands are similar in that effects of dredge. 1. Result in a much decreased biomass of the target species, numerical reductions and likely decreased biomass of non-target species. 2. Are much more pronounced in areas with diverse communities and stable environmental conditions have some effects on certain types of sediment and can change sediment parameters at least in the short term. 3. Depend on the time of year the cockle bed is being exploited will be most severe if sufficient recovery time is not allowed. Results from this study did not agree with the conclusion that recolonisation takes place fully and quickly from nearby areas. Effects were obvious at Site A even at the end of the experiment.

General effects on birds. Reductions in Hydrobia ulvae populations could affect shelduck, knot, dunlin and redshank. Disturbances to bivalve molluscs could affect oyster catcher, shelduck, knot, curlew and eider ducks, the latter however preferring M. edulis. Polychaetes are important in the diet of curlew, dunlin, bar tailed godwit and redshank although the latter prefer Hediste from the upper shore regions. Amphipods figure prominently as food for dunlin, curlew, oystercatcher, knot and shelduck.

Location

Burry Inlet - Loughor Estuary (Llandhidrian sands)

Fishing Types

Cockle tractor dredge. Disturbance (general).

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Norris, K., Bannister, R.C.A. & Walker, P.W 1998. Changes in the number of oystercatchers Haematopus ostralegus wintering in the Burry Inlet in relation to the biomass of cockles Cerastoderma edule and its commercial exploitation. Journal of Applied Ecology., 35, 75-85

Description

The Burry Inlet supports both a commercially cockle fishery and up to 22,000 oystercatchers, particularly during the winter months. Changes in the number of oystercatchers using the estuary in both winter and spring in relation to biomass of cockles present at the start of winter and then removed through the fishery were assessed.
Oystercatcher population: counts were carried at each month (by one volunteer to avoid observer error) from 1974 to 1993.
Cockle population: 15 transects, (400m apart with sample station every 50m) running due magnetic north were set up across the cockle bed. Samples collect from 0.1m2quadrat and substrate removed to 6cm depth, sieved through 4mm sieve and number of cockles counted.

Habitat Effects

Species and Community Effects

Cockle biomass varied considerable between winters, 17,500 tonnes in 1983/84 declining to 7,350 tonnes in 1991/92. Winter landings indicated less variability over time than biomass, but landings have increased over time reaching a peak in 1992/93 of 2300 tonnes. Multiple regression analysis gave no evidence that winter bird-days varied in relation to cockle biomass or winter landings. Indicating that the number of oystercatchers overwintering on Burry Inlet was not determined by cockle stock at the beginning of the winter or by the biomass removed by the fishery. However, spring bird-days did vary in relation to cockle biomass and winter landings. As cockle biomass increased so did the number of spring day-birds but, the number spring day-birds decreased as the number of winter landings increased. So, although the number of oystercatchers arriving on the estuary in winter is independent of fishery landings and cockle biomass the number remaining in the spring appears to dependant on these variables.

Experimental investigation on the effects of cockle dredging on spat settlement using a 71 cm mechanical dredge with revolving riddle.

Habitat Effects

Species and Community Effects

A single pass of the dredge reduced both fishable and juvenile stocks of cockles substantially. Adult cockles more damaged by dredge than juveniles. No subsequent difference in cockle mortality between dredged and undredged plots. New spat settlement not affected.

A tractor-towed cockle dredge was used on both muddy sand and clean sand, intertidal areas to extract cockles. The effects of dredging on invertebrates and their predators were examined.

Habitat Effects

Species and Community Effects

A significant proportion of the most abundant species was lost from both sites. In muddy sand, populations of Pygospio elegans and Hydrobia ulvae remained significantly depleted for more than 100 days and had not recovered 174 days after harvesting. Some species of polychaete and amphipod remained depleted for more than 50 days. Although bird feeding activity of gulls and waders increased for a short period following dredging due to increased food availability, this was followed by a significant reduction of bird activity compared to control areas. For curlews and gulls, this reduced level of activity continued for 80 days and for oystercatchers, 50 days. In the area of clean sand, invertebrate communities were less dense and recovered more quickly.

Location

Burry Inlet, South Wales

Fishing Types

Cockle tractor dredge. Competition for resources (general).

Reviewed by

Sewell & Hiscock 2005

Full Reference

Norris, K., Bannister, R.C.A. & Walker, P., 1998. Seasonal changes in the number of oystercatchers Haematopus ostralgus wintering on the Burry Inlet in relation to the biomass of cockles Cerastoderma edule and its commercial exploitation. Journal of Applied Ecology, 35, 75-85

Description

Study examining the relationship between biomass of cockles taken by a small scale, hand gathering fishery, cockle biomass and oystercatcher abundance. The study is based on data covering 11 winters.

Habitat Effects

Species and Community Effects

Winter oyster catcher numbers were not correlated with cockle biomass nor biomass taken by the fishery but with the total number of overwintering oystercatchers in the UK overall. Spring oystercatcher numbers were however positively correlated with cockle biomass and negatively correlated with cockle biomass extracted by the fishery. The authors believe that the reason for this is that oystercatchers leave the area earlier in spring when biomass at the start of the winter is small and/or the biomass extracted by the fishery is large.

Experimental tractor dredging for cockles was carried out on a cockle bed, previously only harvested by hand raking methods. Six boxes of dredged and control plots were set out in each of two areas, one with high and the other low densities of cockles.

Habitat Effects

Approximately 82 percent of the dredged area was lifted by the blade of the dredger.

Species and Community Effects

Catch consisted almost entirely of adult cockles over 2.5 cm in length. Appreciable losses of smaller cockles and spat were also observed in dredged areas. Spatfall success in 1993 was depressed by 11 percent on dredged plots compared to that on control plots in the low density area, but was increased slightly in the high density area. Delayed effects of the dredging on cockle stocks were thought to be negligible.

Location

Burry inlet, south Wales

Fishing Types

Cockle tractor dredge.

Reviewed by

Sewell & Hiscock 2005

Full Reference

Rimington, N, 2002. The relationship between mussel and oystercatcher populations in the Burry Inlet, Part 2 Countryside Council for Wales. CCW Contract Science Report No 491

Description

Study using a modelling approach to identify how the removal of 'mussel crumble' from a cockle bed in the Burry Inlet would affect overwintering populations of the oystercatcher Haematopus ostralegus. The study aimed to identify whether birds in the inlet are food limited under current conditions and to predict any ornithological implications of changes in shellfishing activity.

Habitat Effects

Species and Community Effects

The simulations indicated that removal of 'mussel crumble' and a change to a cockle fishing regime would be unlikely to effect oystercatcher numbers. It was also suggested that fishing practices that reduce shellfish numbers, but do not reduce the area covered by shellfish beds are less likely to have a negative effect on bird populations than fishing practices that reduce the area covered by shellfish. This is due to the increased bird density and interference competition that may occur as a result of reduced shellfish-bed area.

Study, reporting the effects of beam trawling on the in- and epifauna associated with the biogenic structures (tube heads) formed by the overlapping tubes of serpulid worms (Pomatoceros triqueter and Pomatoceros lamarcki). The site was trawled at six monthly intervals for two years. Samples were taken using a quantitative epibenthic dredge.

Habitat Effects

Species and Community Effects

No significant changes to the distribution and number of tubeheads attributable to fishing were detected. No significant changes in community structure associated with the tubeheads was recorded. An additional laboratory study suggested that following disturbance, tube heads are unlikely to return to their original positions on the sea bed.

The study examined how the behaviour oystercatchers changed in response to experimentally applied sub-dispersive human disturbance. The following hypotheses were tested: 1) disturbance will increase walk rates, 2) disturbance will increase the amount of time that birds spend vigilant and 3) as a result of the disturbance the foraging efficiency would be reduced. During spring 1999 observations were made for 5 minute periods on the following: walk rates, feeding rates, vigilance scans and foraging success. Information on weather conditions, incidental disturbance and group size was also recorded. All observations were made through a 60x telescope and all behaviours were dictated into hand-held tape recorder.
The experimental disturbance was caused by an assistant working to and from the foraging flock, initially once then two to four passes in 5mins. One disturbance trial was carried at low tide once a day-the level of disturbance applied was randomly selected.

Habitat Effects

Species and Community Effects

As a result of the disturbance the speed at which the oystercatchers walked away from the disturbance almost doubled, but the amount of time spent walking was not affected. Disturbance also increased the number of scan events and their average length, this therefore meant that the amount of time in-between scans for foraging decreased. The frequency of the disturbance did not affect the number of feeding attempts – there was no difference in the number of feeding attempts made between birds that were disturbed and those that were undisturbed.

Study examined the effects of specific fishing gear used in Canadian waters, this included otter trawls, scallop dredges and hydraulic clam dredges and their effects on seafloor habitats and on populations and communities of benthic species.

Habitat Effects

The main points from the study indicated that bottom gear can damage or reduce habitat complexity and structural biota, as well as altering the structure of the seafloor and large habitat features (could have positive or negative consequences). The initial impact tends to be greater on sandy and muddy bottoms than on hard, complex bottoms; however, the duration of impacts tends to be greater on hard complex bottoms. Dredges and bottom trawls are also considered to cause the most damage to benthic populations, communities and habitats per unit effort.

Species and Community Effects

Bottom gear can impact benthic populations and communities by changing the relative abundance of benthic species, which in turn can alter the composition of benthic communities. Bottom gear can decrease the abundance of long-lived species and increase the abundance of short-lived species.

Potential threat to red-throated divers from gill and tangle nets high. May have knock on effects at the birds breeding grounds. During 14 inspections of beach set nets between September 1991 and December 1992 no seabird by-catch was noted despite red-throated divers observed diving within 20m of nets.

Ten harbour porpoises Phoecoena phocoena reported as casualties of gill nets in 1991. Author considers that Harbour porpoise is the only cetacean under severe threat of extinction from static fishing gear in Cardigan Bay. 24 percent of UK deaths of harbour porpoises caused by entanglement in fishing gear.

One Grey Seal Halichoerus grupusfound stranded in 1991 with injuries consummate with gill net entanglement. Net inspected in September 1992 no by-catch recorded despite close proximity of grey seal. Young seals more likely to suffer from entanglement. Juvenile dolphin recorded tangled in net. Author concludes no major entanglement problem in Cardigan Bay.

Assessment of cetacean by-catch in the Irish and UK set gill net fisheries for hake in the Celtic Sea over 19 months based on observer programme.

Habitat Effects

Species and Community Effects

Marine mammal by-catch during the sampled trips was 43 porpoises and 4 common dolphins. One porpoise was in a tangle net the rest in the hake nets. No relationships were recorded between by-catch rate and water depth and no significant differences between hake nets with double or single footropes. There were significantly higher by-catch rates during neap tides but no correlation with sea state during net hauling or with hake landings. Observations consistent with porpoise entanglement occurring while net is one the bottom. By-catch rate was 7.7 porpoises per 10,000 km/hr of net immersion.

Fishery scientists accompanied Irish, commercial pair trawlers, targeting herring in the Celtic sea. Net openings were 15-20m high and 20-30m wide. Bycatch was examined and the number of large marine mammals present in trawls were counted. During the study, 78 tows took place and a total of 101 hours of towing time was monitored. Nets were towed at depths of 14-55 m in water depths of 24-75 m.

Habitat Effects

Species and Community Effects

Single grey seals, Halichoerus grypus were captured and killed in four trawls. This equated to 0.05 seals per tow or one seal per 317.5 tonnes of fish caught. From this, the authors conclude that approximately 60 individual grey seals are captured in the herring pair trawl fishery per year. The authors conclude that due to the timing of the herring fishery in this area, it is not likely to cause a decline in the Irish grey seal population. A group of four harbour porpoises were reported during the study, but none were caught in fishing gear.

An estimation of seabird by-catch from a fishery consisting of several fleets in central North Pacific. The assessment method was based on by-catch observation data from one fleet, following this scenario analysis was used to estimate the by-catch for the rest of the fleet, best and worst case scenarios were also provided.

Habitat Effects

Species and Community Effects

Population trajectories suggested that even with the best-case mortality level, 1.9% (5,200 individuals /year) of the population would be killed by pelagic long-lines each year, declines in the population would be likely over the next 20 years. The worst case scenario suggested that as many as 10,000 individuals are killed each year.

Location

Central North Pacific

Fishing Types

Longline.

Reviewed by

Sewell et al 2007

Full Reference

Caddy, J.F., 1973. Underwater observations on tracks of dredges and trawls and some effects of dredging on a scallop ground. Journal of the Fisheries Research Board of Canada, 30, 173-180

Description

Trials looking at effects of three types of trawling gear on bottom sediments. Shallow traces made by inshore and offshore scallop dredging could be distinguished from each other.

Habitat Effects

Scallop dredging observed to lift fine sediments into suspension, bury gravel below the sand surface, and overturn large rocks embedded in the sediment, appreciably roughening the bottom. The inshore Alberton dredge was inefficient, dumping its contents back on to the bottom at intervals. Trawl tracks were seen as grooves on the seafloor - considered to be made by otter trawl doors. Suspended sediment in dredge tracks reduced visibility from 4-8m to less than 2m within 20-30m of the track but dispersed within 10-15mins, coating the gravel in the vicinity of the track with a thin layer of fine silt and obscuring Lithothamnion. Offshore dredge - gravel fragments overturned. Depressions left by tow bar of the dredge. Gravel less frequent inside the track. Inshore dredge (Alberton) tracks left, gravel sparser inside and dislodged boulders commonly observed. Tooth marks over sandy bottom.

Bottom type and hydrographic regime in the Bay probably allowed marks made by fishing gear to remain recognisable for a long time as tidal currents faster than 1km/hr were not encountered. Even a relatively minor fishery may therefore have a significant cumulative effect on bottom microtopography under these conditions. Scallop and otter tracks could be distinguished, scalloping contributing to an appreciable roughening of the bottom, lifting large boulders and overturning many of them, presumably leading to destruction of the epifauna on their upper surfaces. Under strong tidal flow author considers that intensive dredging will lead to erosion of sediment lifted into suspension by the dredge - this aspect needs more study.

Species and Community Effects

Dredging caused appreciable lethal and sublethal damage to scallops left in the track. Damage greatest on rough bottom. Predatory fish and crabs were attracted to dredge tracks within 1hr, and fish were observed in the tracks at densities 3-30 times those observed outside the tracks. There was a pronounced and rapid aggregation of foraging fish - a natural response which also occurs in the absence of fishing operations.

The work was a comparison between maerl and associated benthos in regularly fished and unfished areas, both before and after dredging with a 77 cm diameter Newhaven scallop dredge. The study included comparison between maerl thalli collected in the late 1800s and the study date from a separate site, which had been extensively dredged for the prior 40 years.

Habitat Effects

A 2.54 m wide track with three parallel furrows was created at test sites in both areas. All natural, physical bottom features were eliminated and boulders of up to one cubic metre had been dragged along the surface. Sculpted ridges made by the trawl were still apparent after 2.4 years at the previously undredged site and 1.5 years at the previously dredged site.

Species and Community Effects

The scallops Pecten maximus were more abundant at the unfished site. File shells Limaria hians and their nests and the scallop Aquipecten opercularis were present at unfished sites, but not at fished sites. Immediately following the trawl, live maerl was buried and biogenic structures were crushed and destroyed. There were no signs of recovery of maerl within the four year study.

Results from a consultation exercise involving trawlermen and creel fishers of the Clyde Sea. A questionnaire was used to identify interactions between fishers and common and grey seals.

Habitat Effects

Species and Community Effects

91 percent of Trawlermen reported catching a seal in their towed gear rarely or occasionally. The majority of these were reported as being dead when recovered. This was compared to only nine percent of trawlermen reporting damage to their gear by seals. There did not appear to be any reports of seal mortality caused by creel fishermen.

Study to quantify impacts of hydraulic blade dredging for razor clams. The study focused on discard generation, damaged caused to the catch and the ability of disturbed organisms to rebury following disturbance.

Habitat Effects

Species and Community Effects

Dredge contents and dislodged fauna were dominated by the heart urchin Echinocardium cordatum. Approximately 80 percent of these survived the dredge proccess. The majority of heart urchins left in the dredge track that were undamaged were able to rebury following the disturbance. However, none that were brought to the surface after dredging were unable to succesfully rebury within three hours of being returned. The second most common species were the target razor clams Ensis siliqua and Ensis arcuatus, as well as the otter shell Lutraria lutraria. Of these, between 20 and 100 percent of those caught suffered severe damage in any one haul. Approximately 85 percent of razor clams were able to rebury following disturbance.

Study examining the effect of dredging for scallops at previously fished and previously unfished maerl beds. Fishing took place using a gang of 3 Newhaven dredges with 77 cm mouth width. The impact on benthic species was measured, as was bycatch in the dredges. The dredge sites were monitored immediately after dredging and four times a year for the following four years.

Habitat Effects

Direct observations showed profound 2.5 m wide tracks were made through the maerl beds, in which, all natural bottom features were erased. Rocks and boulders were overturned, sediment was brought to the surface and live maerl was buried.

Species and Community Effects

During the trawl a number of large and fragile species were killed or damaged by the trawl. This included damage to individuals and nests of the file shell Limaria hians. Investigations immediately after the dredge revealed littering of animal fragments and damaged animals across the seabed. This was followed by an influx of opportunistic scavanging species, that began to disperse after three days. Different groups of organisms recovered at different rates over the four years of surveying after dregding. Large, slow-growing bivalves such as the horse mussel Modiolus modiolus and the file shell and some sponges and anemones had not recovered after four years. File shells, their nests and diverse associated fauna remained absent for the duration of the surveys.

Most were drowned foraging in depth of 2-5m of water. Greatest depth was 15m, 65 percent of known status were adult females 15 percent were juveniles, 10 percent sub-adult females and 10 percent adult males. The low number of males perhaps because fewer adult males in the favoured breeding area. Also because of their size the males may not be able to enter the parlour of the creel. Fish such as saithe, small cod and congers swim into the creels and are trapped and it is likely that the otters are attracted to this rather than the lobster bait.

The incorporation of a parlour in these pots has greatly increased its ability for holding lobsters as well as otters. Does not appear to be as much a threat from crab creels as they are usually set on sandy bottom in deeper water further offshore rather than the favoured otter foraging areas.

The detection range of harbour porpoises and the bottle-nosed dolphin for 11 different types of gill net was estimated based on calculated target strength of each type of net and known echolocation abilities of each species.

Habitat Effects

Species and Community Effects

The study suggests that echolocating bottlenose dolphins can detect nets in time to avoid collision, whereas echolocating harbour porpoises cannot in most cases.

Study examining the conflicts between grey seals and fishermen in a Cornish trawl fishery. The report examines the 'economic value' of seals and includes analysis of the impact of fishing on the local grey seal population.

Habitat Effects

Species and Community Effects

About 80 individuals belonging to the Cornish Grey Seal population (of about 400 individuals) are killed as a by-catch of trawling annually.

Investigations by the author into numbers of dead seabirds on the shore in early 1970s at Cruden Bay in NE Scotland in mid summer.

Habitat Effects

Species and Community Effects

Study led to conclusion that seabirds must have been killed in some of the numerous local fixed salmon nets which were often seen holding dead birds. Most were auks which are known to be killed in fixed salmon nets on a considerable scale around the seabirds colonies on St. Abbs Head and Troup Head in the Moray Firth. Some shags also reported killed in nets set near a roost on the Summer Islands. Off the Scottish Wildlife Trust reserves at Longhaven and on the Dunbuy Rock to the south up to 17 bodies per net were recorded on the 12 or so occasions they were examined during the breeding season over the previous four years.

Experimental study examining the effects of scallop dredging on the fauna and sedimentary characteristics of a silty sand community. A 2 m wide Bedford-style scallop dredge was dragged 23 times across the study site and this area was compared to an a undisturbed, adjacent site. The two areas were sampled four and five months before, immediately before and after and four and six months after dredging.

Habitat Effects

Passing the dredge over the site removed the surface few centimetres of sediment. Food quality of the sediment was reduced, as was calculated by measuring microbial populations, enzyme hydrolysable amino acids and chlorophyll a levels. This reduced food quality showed relatively complete recovery within four to six months.

Species and Community Effects

Immediately after dredging, macrofauna were significantly decreased in overall abundance and assemblage structure was altered at the dredged site. Macrofaunal abundance and assemblage structure at the dredged site did not recover to levels equivilent to the undredged site before six months.

Report on deep sea coral reefs. The relevant section, discusses the main threats to coral reefs, including fisheries. The main types of fisheries that operate over deep-water coral reefs and their impacts are discussed.

Habitat Effects

The main conclusions from this section of the report are: Bottom trawls beam and otter trawls operating over coral reefs can smash, disrupt, tear, break and effectively flatten coral reefs, reducing the structural complexity of the habitat and reducing the number of associated species. Further damage can also be caused by the resuspension of sediments.

Dredges The effects of dredging for bivalves over deep-water corals are similar to those caused by trawls.

Bottom-set gillnets Physical damage can be caused to the reef by anchors and weights and lost nets (ghost fishing) can continue to catch fish for years after they are lost. In Norway, attempts to retrieve these nets have used gear that is damaging to coral reef areas.

The failure of methods to control oyster drills led to the development of the hydraulic suction dredge, however the effect of the hydraulic suction dredge had not been evaluated on anything other than the drills and oysters themselves. This study aimed to assess the effect of the hydraulic suction dredge on the oyster community and the sedimentary composition of the bottom. Three sites were selected i) Laboratory Ground, ii) Ground 515 and iii) Ground 154 at each site there were control and test plots with five sampling stations at each. To assess the effects of the hydraulic suction dredge on benthos two techniques were used: Peterson grab for the soft-bottom site (Ground 154) and a suction sampler on the sites where there was an abundance of shells (Laboratory Ground and Ground 515). To assess the effects of the hydraulic suction dredge on sediments two techniques were used: Peterson grab was used at the Ground 154 site and an Orange-Peel sampler was used at both the Laboratory Ground and Ground 515 sites. Samples were collected before and after dredging.

Habitat Effects

The sediment at the selected oyster sites were a mixture of fine and very fine sand with silt-clay, can be described as muddy sands. Immediately after dredging there was a reduction in median grain size as a result of additional silt-clay being brought up from the subsurface layers on the Ground 154 test plot. In contrast at the Ground 515 test plot there was a slight increase in median grain size as a result of a slight loss in the silt-clay content.

Species and Community Effects

A total of 289 benthic samples were collected before and after dredging and were examined for benthic fauna.
Changes in numbers of oyster drills (target species): On the Laboratory Ground the number of oyster drills was reduced by 85% on the test plot and 34% on the control plot immediately after dredging. After 10 months the number of oyster drills on the test plot had increased by 50% but this was still less than half the number on the control plot. On the Ground 515 site the number of oyster drills on the test plot was reduced by 79% immediately after dredging, but 3 months later the numbers on the test plot had increased 9-fold and were larger when compared to the control plot.
Changes in total numbers of animals: At the Laboratory site the average density of animals was significantly reduced immediately after dredging (70.3% at the test plot and 72.4% at the control plot), however the difference came from the high density of mysid Neomysis americana present at both sites before dredging. If Neomysis americana was excluded there was a decline, but this was not considered to be significant (59.1% reduction at the test plot and 29.4% at the control plot). Recovery was not considered to be rapid either, 10 months after the dredging density was still lower on the test plot than the control plot. The situation was similar at the Ground 154 site, only with a different species (Polydora ligni). When included there was a 43.2% reduction, when excluded there was a 16.9% reduction in density at the test site. However, at the Ground 515 site there was a significant increase in the density of animals at the test plots immediately after dredging (increase of 74.6%), this was explained by a heavy larval recruitment of the sand shrimp, mysid and spionid poltchaeta. On the control plots there were no density measurements before dredging, but after immediately after dredging the density on the control plot was higher than the test plot. Three months after dredging the density of animals on the control and test plots were similar, which indicated a complete recovery of the animals at this particular site.

Report of catches off Devon coast, off the east coast of South Uist, Orkney, Skye, Shetland and west Sutherland

Habitat Effects

Species and Community Effects

Accidental drowning of otters has occurred in crustacean and fish traps such as lobster pots, crab pots, and eel fyke nets in both freshwater and marine situations. Review of reports shows that this has taken place in parlour creels, single-compartment box creels, single compartment ?inkwell? creels and fyke nets. Work to prevent otter damage to fyke cod-ends suggests that in some cases they attack the nets from the outside and if severing the mesh proves impossible, move to the fyke entrance or directly to the entrance. Uncertain whether otters are attracted to crustacean traps by the bait or the catch -seems that both can occur. In the latter case this is because they tend to contain particularly favoured prey such as eels, crayfish and crabs. Estimates of times otters can submerge are for more than 3-4 mins, normal dive time is far shorter and they run out of time and drown. Sex and status of otters drowned in lobster creels off S. Uist mostly females. Adult males may be less active in the favoured breeding areas and may be unable to enter the parlour of the most widely-used creel. No data to support the view that those otters which drown are young and inexperienced. Some evidence to suggest that they escape more readily from single-compartment creels than double-chamber creels. Family parties are known to have drowned on five occasions. Juvenile casualties have involved animals towards the size where independence is reached, at about 10 months.

Location

Devon coast, east coast of South Uist, Orkney, Skye, Shetland and west Sutherland

The study was undertaken to assess the cockle resource in Dundalk Bay as the area is both an SPA for overwintering birds and a cSAC (candidate Special Area of Conservation). 14 samples were collected between October 2003 and October 2004 using a continuous delivery dredge. A survey of cockle densities were also collected on the North Bull (March 25th to April 2nd 2004) and South Bull (April 29th to June 10th 2004) by raking a 1m2 randomly-chosen quadrat over the extent of mud and sand.

Habitat Effects

Consequences of mechanical dredging other that the amount of damage caused to the target species were not investigated.

Species and Community Effects

The cockle population in the Dundalk makes up 99% of the biomass of bivalves on the mud and sand flats. The population was shown to be short lived, with 7.42% of the discarded cockles showing signs of damage. Although the population density was shown to be lower than that of the Wash or Burry Inlet, the current fishery in Dundalk may be the largest in the area since 1970. The Dundalk was designated an EMS based on primarily the presence of oystercatchers, although population counts may vary it was calculated that oystercatchers in Dundalk might consume 1,400 tonnes of cockles between October and March, in spring 2004 the biomass in Dundalk Bay was estimated at 1,645 tonnes.

Assessment of the potential changes to macrofaunal community structure beneath trestles and in access lanes associated with large scale oyster culture. Two different treatments were sampled, the first directly under the oyster trestles (possible subjected to additional organic input from oyster faeces) and the second in access corridors (potentially subjected to heavy vehicles). 300m away a control site was established. 10 samples were collected from each treatment.

Habitat Effects

Species and Community Effects

In total 21 macrobenthic species were recorded, but only 9 showed a significant difference between the three sites. At the control site high densities of Nephtys hombergii, Bathyporeia guilliamsoniana, Gammarus crinicornis, Microprotopus maculatus and Tellina tenuis were recorded when compared with the other two sites. In the Trestle treatment higher densities of Capitella capitata were recorded and in the Lane treatment higher densities of Scoloplos armiger, Eteone longa and Sigalion mathildae were recorded. When the lane and trestle treatments were compared to the control treatment, the lane treatment indicated a slightly higher diversity value and the trestle treatment showed a slightly lower diversity value.
Composite Abundance Biomass Comparison (ABC) curves (based on mean abundance and biomass values) for both the trestle and control treatment suggested a relatively undisturbed community (biomass curve above abundance curve). However, in the lane treatment the ABC curve suggested a more disturbed community (abundance curve above biomass curve), likely to be caused by heavy vehicle traffic for maintenance and harvesting.
Results indicated that the benthic communities underneath the trestles have not undergone any form of organic enrichment as no elevated levels of organic matter or potential enrichment indicator species were recorded.

Study looks at the changes that mechanical cockle dredging has on the quality and quantity of food for the red knots and how these changes effect the survival of these birds. The prey available was sampled from late July to early September each year (1998-2002). At each station (in total 2,846 stations of which 75% were sampled every year) a core sample was taken and the contents sieved over a 1mm mesh, all potential prey items were frozen and taken to the laboratory for identification. Dredging took place from September to December and the effects on cockles that were actually available to the knots were analysed. The diets of the red knots were assessed through faecal analysis.

Habitat Effects

Species and Community Effects

Effects on target species: Within the dredged areas the density of small cockles remained stable however in the undredged sites the density of small cockles increased by 2.6%. There were also differences in the quality of the cockles between dredged and undredged sites. In the undredged areas the quality of the cockles remained stable, but in the dredged areas the quality declined by 11.3%.
Effects on non-target species: As a result of decreases in the quality of the cockle stocks the red knots were shown to increase their gizzard mass to compensate for this decline. However, this was not sufficient and resulted in a decrease in local survival rates, indicating the effect that fishing can have on bird populations.

Paper looks at the fishery policy that has come into force since 1993 and the effects that low food stocks have had on oystercatchers and eiders, which depend on cockles and mussels.

Habitat Effects

Species and Community Effects

In the early 1990s low food stocks resulted in food shortages for both oystercatchers and eiders, from this food shortage three main effects could be seen: i) effects on numbers, ii) effects on distribution and iii) effects on mortality.
Effects on numbers: in January 1991 the number of eiders was 35% lower than the average number between 1970 and 1990, in the following 1992/93 season the number fell to around 64,000 birds, this figure was considered to be half of the ‘normal’ number of birds. Low numbers of oystercatchers were also recorded in 1991, but from 1992 to 1995 there was a recovery period. However, after a cold winter the number of oystercatchers fell sharply, when most of the cockle died, numbers dropped further in 1997.
Effects on distribution: during the winter of 1990/91 a high number of eiders were recorded in the North Sea, this was something that had not been since before, at the same time numbers in the German Wadden Sea also increased. Despite increases in other areas the number of eiders in the Dutch part of the Wadden Sea has decreased since 1990. In the 1990s a large number of oystercatchers were recorded inland and in some case 100s km or more away from the Wadden Sea. The mild winters of 1991/92 and 1992/93 may have allowed for inland feeding.
Effects of mortality: the number of beached eiders has increased slowly since 1989 and peaked in 1992 with over 19,000 found dead. High oystercatcher mortality occurred in 1987 due to a combination of cold temperatures and low food stocks.
Large scale bird counts and habitat characteristics in the Dutch Wadden Sea have indicated that the distribution of oystercatchers in this area are partly controlled by the distribution of intertidal mussel beds. A similar situation was evident on the island of Ameland for both oystercatchers and eiders, where the number of overwintering oystercatchers increased from 1975-1990, during this time the area of intertidal mussel beds also increased. However, in 1990 the area of intertidal mussels disappeared and the number of oystercatchers dropped, by the mid 1990s the mussel beds had started to recover and the number of oystercatchers increased. Eiders in the same area showed the same pattern.

Beukema, J.J., 1995. Long-term effects of mechanical harvesting of lugworms Arenicola marina on the zoobenthic community of a tidal flat in the Wadden Sea. Netherlands Journal of Sea Research, 33, 219-227

Description

Study of the effect of mechanical lugworm dredging on the macrozoobenthos of a large intertidal mudflat. A transect, being used for a long-term sampling programme, established in 1970, was crossed by mechanical lugworm dredgers from 1978, up to and including 1982. Sampling continued during and after this dredging and findings are discussed.

Habitat Effects

Species and Community Effects

A severe reduction in lugworm stock occurred during the dredging period, Total zoobenthic biomass also declined. The population of gaper clams Mya arenaria reached almost complete extinction when, prior to dredging, the species had comprised half of the total biomass. Recovery of the biomass of the zoobenthos took several years. Lugworm stocks recovered slowly and reached original levels after three years. The Mya arenaria population took five years to recover to original density levels.

The study was carried out following unusually high mortality of common eiders in 1999/2000 (approximately 21, 000 birds died). The area surveyed was home to an intense cockle fishery and mussel culture, both reducing the principal food source for the common eider. Dissected eiders showed signs of starvation.

Habitat Effects

Species and Community Effects

Cockle biomass was extremely low during the winter of 1999/2000 , however fishing continued. The remaining cockles were low quality. Stocks of Spisula clams in the North Sea (a secondary food source for the common eider) were heavily fished during the end of the summer of 1999 resulting in a loss of 85 percent of stock in some areas. The authors conclude that the likely cause of death of the eiders was starvation, resulting in a lack of food caused by overfishing of the eider's principal and secondary food sources.

The study aimed to assess the variability of cockle production and the extend to which contributing factors affect the annual and local production values. Finally, annual estimates of cockle production and biomass were considered against the food requirements of the oystercatcher to determine whether or not a sustainable fishery is possible on the tidal flats of the Dutch Wadden Sea.

Habitat Effects

Species and Community Effects

The production of cockles on the tidal flat has been variable during the period of observation. The variability of recruitment seems to be governed by factors relating to the character of the foregoing winter, in particular the presence of epibentic predators (shrimps) on the tidal flat during the early benthic stages of cockles. Future prospects for high cockle production and biomass in the Wadden Sea appear to be poor. This will have consequences for wintering birds in the Wadden Sea that have a specialized diet dominated by large bivalves like cockles and mussels. Due to the low levels of cockle recruitment success during the last 15 years and the poor prospect for the development of future strong cohorts any cockle fishery in the Wadden Sea may be considered harmful to wintering birds.

The effectiveness of MPAs for protecting oystercatcher populations was investigated in the Dutch Wadden Sea. 520 oystercatchers at 7 sites were captured with cannon nets or mist nets where biometric parameters were measured following standard techniques these included bill tip shape, sex, age as well as blood samples being taken.

Habitat Effects

Species and Community Effects

There has been a decrease in the number of wintering oystercatchers in the Dutch Wadden Sea over recent years (250,000 to 150,000). It was thought that with the introduction of MPAs to protect the oystercatchers’ food source this species would redistribute itself in relation to food supply. However, there has been no indication that this has occurred and oystercatchers have not increased in numbers within the MPA.
There was no difference in the body size or age of oystercatchers between the protected and protected areas. The bill tip shape however did indicate differences; the number of oystercatchers inside the protected areas with a ‘shellfish tip’ was much higher when compared to the number outside the protected areas (40.3% n = 4 sites, vs. 24.5% n = 3; p < 0.02). These differences were similar between males and females; the number of oystercatchers eating shellfish was much higher in male populations. This indicates that male populations are more vulnerable than females to low shellfish stocks. Oystercatchers in the unprotected sites were found to have a much lower level of shellfish within their diets and the estimated mortality was 43% higher. For this reason it is therefore likely that shelfish fishing can explain or at least partly explain the 40% decline in oystercatcher numbers over recent years.

The study compares benthic macrofauna between the unfished Crab and Halibut Protection Zone 1 (CHPZ1) and heavily fished areas where bottom trawling is known to take place. Sampling was carried out using otter trawls.

Habitat Effects

Species and Community Effects

Sedentary macrofauna were found to be more abundant in unfished areas. Overall diversity and niche breadth of sedentary taxa was higher at unfished sites. Within groups of motile organisms, a variety of responses were observed and the authors suggest that this may indicate the importance of life history characteristics such as habitat requirements and feeding mode.

The aim was ‘to assess the utilization of discarded fish by scavenging seabirds behind trawlers in both areas’. Within the study areas data from 28 commercial trawls was collected, 15 from the Ebro Delta and 13 from Majorca. Stern counts were carried out in a 360º scan around the ship; species that followed the vessel every 15 minutes from the beginning of the haul to the end of discarding were recorded. Of the fish discarded, species, length, whether it sank or was picked up by a bird was recorded.

Habitat Effects

Species and Community Effects

Gulls and terns followed behind the trawlers, Procellariiformes were noted away from the stern Skus moved around the trawlers, but often kept away from them. Results suggest that discards from trawler fleets readily support all scavenging seabirds breeding at Ebro Delta, but not at the Balearic Archipelago. As discards from the Balearic Archipelago are unable to support the energy requirements of scavenging seabird populations, as a result of the competition for resources and suitable breeding sites it may explain why populations of the Audouin’s gull increase at a much lower rate in the Balearic Archipelago than the Ebro Delta.

Life history of 24 species of sturgeon summarised with details of the three different life histories depending on whether the adults remain in fresh water, move into brackish water or finally move into the sea.

Habitat Effects

Species and Community Effects

Location

Europe

Fishing Types

Various (see further notes).

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Northridge, S., di Natale, A., Kinze, C., Lankester, K., Ortiz de Zarate, V. & Sequeira, M., 1991. Gill net fisheries in the European Community and their impacts on the marine environment MRAG Ltd. A report to the European Commission's Directorate General Environment..

Around the UK catches of grey seals in tangle net fisheries high in the Barra fishery and for Cornwall appeared to be higher than other areas. Catches of common dolphins often reported in southwest fisheries amounting to perhaps some hundreds per year. Bottlenose dolphins rarely recorded but porpoises fairly frequently found in gill net fisheries especially in the North Sea. Drift net fisheries catch most but most of these are released alive. Total drownings in gill nets throughout the country may be in high tens to low hundreds. Impact on porpoise population not known. Bird catches widely reported but little studied. Catches of non-target fish poorly known but crabs are taken in very large numbers.

Regarding impact on marine mammals the study clarified importance of North Sea cod fishery and Atlantic hake fishery both already suspected of taking significant number of harbour porpoises and common dolphins respectively. With no populations studies on this species in Europe the impacts of these fisheries and the recently implemented tuna drift net fishery, remain speculative. There are apparently significant catches of birds in the salmon driftnet fisheries in Ireland and Denmark and catches in coastal and lagoon fisheries in Portugal and Italy. It has been estimated that breeding populations of guillemots at two sites in northern Norway have declined by 95 percent from the early 1960's to 1989 and that this decline could be explained entirely by gill net mortalities based on observed catch rates.

Impacts on non-target fish poorly documented, but where examined a wide variety of species recorded. Probably most acutely seen in the swordfish driftnet fishery. May be an impact on benthic communities because of cumulative effect of exposure to netting (including lost netting) on certain seaweeds, seagrass or pedunculate invertebrate communities may be important but little investigated.

Paper presented a model to test how populations may be affected under new environmental conditions, this is particularly important when the future impact that development of proposed mitigation measures may have on populations. The model was based on the main assumption that an individual within a population will always act in order to maximize its fitness. The model was tested on the oystercatcher feeding on blue mussels in the Exe estuary during the non breeding season and was designed to predict how much of the oystercatchers mortality rate would be affected by environmental changes. The starving number of birds between September and March was the principle determinant for the model. The mortality of oystercatchers on the Exe estuary was measured between 1976 (September) and 1980 (March), the model was the calibrated. The main test of the model was its ability to then predict the starvation rates in a sample of subsequent years (1980-1991).

A behaviour-based was used to evaluate and predict the impact that disturbance could have on the individual survival and long-term population size of oystercatchers (Haematopus ostralegus) in the Exe Estuary. The study also explored if the current levels of disturbance affected fitness and population size, and evaluated alternative policy operations for managing the disturbance. The design of the model looked at the disturbance of birds at their preferred feeding grounds. Different simulations were conducted to assess the effects of various types and intensities of disturbance and the effects of some mitigation methods put in place to reduce the disturbance. The impact of disturbance was measured as the number of birds surviving as well as the percentage of starving birds over one winter at different sizes.

Habitat Effects

Species and Community Effects

The results from the model indicated that for if the same overall area was disturbed a number of small disturbances would be more damaging than and a few large disturbances. The model also indicated that if time and energy costs arising from disturbance were considered then disturbance would actually be more damaging to the oystercatchers than permanent habitat loss. In order to eliminate the predicted population consequences results indicated that this could be achieved by preventing disturbance during the winter when feeding conditions were harder.

Simulations with behaviour-based model for oystercatchers in five areas to assess the amount of shellfish that must remain after harvesting for the oystercatcher populations to be maintained. The model incorporates aspects of shellfishing that affects oystercatchers.

Habitat Effects

Species and Community Effects

Based on the five estuaries modeled so far for oystercatchers to survive from autumn through to spring between 2.5 & 7.7 times the shellfish biomass that will be consumed by oystercatchers must be available in the autumn.

Behaviour-based model was used to look at the effects that current management regimes of a mussel (Exe estuary) and cockle (Burry Inlet) fishery have on the number and survival of overwintering oystercatchers.

Habitat Effects

Species and Community Effects

Currently neither mussel fishery or cockle fishery have caused oystercatcher mortality to be higher than it would be if fishing was absent, indicating current intensities of fishing activity do not significantly affect oystercatchers. However, changes in management practices can affect oystercatcher mortality and population size, these include increasing fishing effort, reducing minimum landing size or increasing daily quota (these effects can be made worse when prey is unusually scarce or periods of cold weather.

Experimental scallop dredging over a sandy bottom, using a modified 1.2 m scallop dredge with a fixed tooth bar, bearing nine 12 cm long and 1 cm wide teeth, separated by 8 cm spaces. The dredge net was removed. The dredge was towed over exactly the same 25 m2 area a number of times for nine days. Samples and observations were collected after 2, 4, 12 and 25 dredges, to measure the effect of different levels of fishing disturbance.

Habitat Effects

Large, visible furrows were created and all previous bottom features (ripples and irregular topography) were wiped out. Large fragments of shell and stone were dislodged. Grooves and furrows created by the dredge were eliminated shortly after dredging. The time taken for this to happen depended on wave action .

Species and Community Effects

Infauna: The infaunal community consisted of bivalves and peracarid crustaceans, Neither taxa showed any significant decrease with dredging disturbance. The biomass of infaunal amphipoda and polychaeta was reduced in all dredged samples, compared to control samples. Epifauna: Insepections of sites following dredges revealed high levels of damage and mortality to large epifauna, including crabs, large bivalves, urchins and sandeels. Overall: The authors conclude that the effect of the dredging experiment was limited to the selective elimination of a fraction of the fragile, sedentary components of the infauna and the destuction of large epifaunal and infaunal organisms.

Location

Firemore, Western shore of Loch Ewe on the west coast of Scotland

Fishing Types

Scallop dredge.

Reviewed by

Sewell & Hiscock 2005

Full Reference

Hall-Spencer, J., 1995. The effects of scallop dredging on maerl beds in the Firth of Clyde. Porcupine Newsletter, 6, 16-27

Description

Preliminary findings of experimental investigation of 3x77cm rock hopper scallop dredges with 9x10cm dredge teeth on each dredge, on maerl beds including visual evidence of impacts.

Habitat Effects

Cobbles and boulders up to 1m3 overturned by dredge mouths or towbar. Dredge teeth penetrated the maerl beds up to 10cm. Cloud of suspended sediment created by trawl.

Species and Community Effects

Large macroalgae torn up. Large animals including highly mobile species such as plaice either mangled, entrained on the bottom or flicked into the dredge bags. Dredge efficiency in terms of catch thought to be 88 percent on maerl beds. Fine sediments eroded, maerl crushed and killed through burial compromising habitat integrity and recovery. Fine sediments deposited over adjacent areas smothering photosynthetic organisms and stressing filter feeders. Micotopographical effects clearly visible 8 months post dredging and number and diversity of sessile fauna and flora reduced. May be a long term shift from K-selected species to R-selected species in response to dredging.

Location

Firth of Clyde

Fishing Types

Scallop dredge.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Hall-Spencer, J., 1995. The effects of scallop dredging on maerl beds in the Firth of Clyde. Porcupine Newsletter, 6, 16-27

Description

Five maerl beds surveyed in the upper parts of the Firth of Clyde. Some information on the impact on maerl habitats obtained from examination of catches during experimental dredge runs. Preliminary findings. Each ground was a focus of high infaunal diversity and biomass consisting primarily of Phymatolithon calcareum.

Habitat Effects

Immediate effects a bow wave of fine particulates suspended ahead of the gear. Bobbins usually rolled along the surface but ploughed into the sediment by up to 4cm when the two-bar was skewed on impact with large boulder leaving trenches of crushed maerl. Cobbles and boulders up to a 1m3 were dislodged and overturned when hit by the tow bar or dredge mouths.

Species and Community Effects

Dredge teeth projected fully into the maerl deposits. Maerl flicked over dredge mouths creating a cloud of suspended sediment in the wake of the bar. Large macroalgae L. saccharina torn up as dredge dragged through the sediment and large animals Echinus, Echinocardium, Luidia, Mya, Ensis, Ascidiella aspersa were either mangled or entrained or flicked into the chain mail bags. Even highly motile elements were caught eg butterfish, plaice, L. depuratur. The dredging has major repercussions for the structure of maerl habitats and associated biota.

The paper looks at how weather affected the daily habitat choice of the Redshanks (Tringa totanus), taking into account the trade-off between predation risk and starvation risk at two habitats (saltmarshes and mudflats), where predation risk by the Sparrowhawks was considered to be higher on saltmarshes. The hypothesis that was tested stated that a riskier habitat would only a chosen when weather conditions meant that the individuals were not able to meet their energy requirements in the safer habitat. The effects of additional weather conditions were also taken into account. Data was collected 3 hours either side of low water between October 28th 2002 and March 1st 2003 from part of the Tyninghame Estuary, additional data on predation risk was collected during the winters of 1989 to 1992. Attacks on, and captures of Redshanks by Sparrowhawks were recorded by one observer and while the feeding habits of the Redshank were collected by another. During each survey day the total number of Redshanks on the saltmarsh and the total number on the mudflat were recorded from the same viewing location. The abundance of available prey in both the saltmarsh and mudflat habitat was assessed by collecting samples and identifying the contents to species level. Note: in the saltmarsh samples 90% of the prey species found were large Orchestia and within the mudflat samples 83% of the prey species found were Corophium.

Habitat Effects

Species and Community Effects

Predation risk: the results indicated that the frequency of the attacks that occurred on the Redshanks by the Sparrowhawks was 21 times higher on the saltmarsh than the mudflat (48 attacks occurred on the saltmarsh compared to only 3 on mudflat).
Energy budget: the amount of feeding time lost as a result of raptor disturbance was greatest on the saltmarsh than on the mudflats. However, Redshanks had a higher energy intake rate on the saltmarsh, which was independent of date, suggesting Redshanks did not increase their intake rate. Date did however have a small effect on the intake rate on the mudflat, intake was higher in mid-winter. The energy budget of the mudflat was significantly lower than the energy budget of the saltmarsh, the Redshanks that fed on the saltmarsh required 43% less feeding time in order to meet their daily requirements than the Redshanks feeding on the mudflats (if the Redshanks only fed on saltmarsh they would have to spend 1.4 ± 0.6h feeding to meet their daily energy budget, whereas Redshanks that only fed on mudflats would have to spend 6.3 ± 0.9h feeding to meet their energy budget).
Habitat choice: the number of Redshanks feeding on each habitat was initially the same, but in mid-winter there was an increase in the number feeding on the saltmarsh. As the starvation risk increased more Redshanks fed on the saltmarsh. Weather also affected the habitat choice of the Redshanks, with a decrease in temperature more Redshanks fed on the saltmarsh – indicating a greater response temperature than starvation risk. During days when the starvation risk was high, Redshanks fed on the riskier saltmarsh as wind speed increased, during periods of low starvation wind speed had no effect on the choice of habitat.

Describes causes of death for two adult (one male and one female) botttle-nosed dolphins found dead in separate locations.

Habitat Effects

Species and Community Effects

lost angling gear was found to have caused mortality in both dolphins. The first dolphin (female) appeared to have eaten a fish, which had been hooked by an angler and still had a length of line attached. This line became wrapped around the base of the larangeal spout, leading to death by asphyxiation. The second dolphin had died following several secondary complications, dirrectly caused by the injestion of a fish with a large amount of line attached

Looking at data over a 16 year period the paper studied various aspects of the breeding ecology of the great skuas in Shetland. How adults responded to changes in the availability of sandeels and how these changes could affect the annual breeding statistics and long-term reproductive potential was also considered.
The relationship between chick diet and sandeel availability was examined, samples were obtained from food that was regurgitated by the chicks during ringing. Pellets of non-digestible material deposited at sites frequented by non-breeders and in breeding territories were also collected.
The amount of time that great skuas spent away from their territories provided an indication as to the availability of prey.
Breeding success was evaluated by marking nests in early May and the eggs laid in the nests were measure. The nests were then visited at regular intervals until the surviving chicks had fled the nests.

Habitat Effects

Species and Community Effects

Diets: Results indicated that during the 1970s and the early 1980s the chicks fed primarily on sandeels, but by 1983/84 the proportion of sandeels in the diets of the chicks fell from 95% to 61%, the change in diet was compensated for by an increase in whitefish. From 1984-1987 the composition of the chicks diets remained fairly constant. In 1987/88 the proportion of sandeels within the chicks diet fell again from 56% to 5%, the proportion of whitefish increased from 42% to 77% and the proportion of bird meat increased from 3% to 18%. The changes that were seen in the chicks’ diets were closely related to changes in the abundance of sandeels.
Adult territorial attendance: Changes to chicks’ diets also correlated with changes in the territorial abundance of the adults. From 1987-1989 the average attendance of adults per territory was 1.5 during both incubation and the first 2 weeks after hatching, this changed to 1.2 adults per territory during the remainder of the pre-fledging period. These results indicated a 50% reduction in attendance from the 1976 breeding season. Possible indicating an increase in both time and effort spent foraging for food.
Breeding success: Results indicated that although clutch size and hatching success did not change between 1976 and 1989, the fledgling success decreased. In 1976 fledging success was 91%, but this decreased to 16.5% in 1989.

The paper looked at the benthic communities found in the Fowey/Eddystone scallop grounds in the English Channel (these have been heavily dredged for more than 10 years) and also looked at relating the distribution of scallops to water depth, community characteristics and substratum type. Scallop dredge survey: at each site two scallop dredges were towed for 15 minutes, benthic fauna from the dredges were brought aboard for analysis and identification down to the lowest taxonomic level, species that could not be identified were preserved in 70% alcohol and identification in the laboratory (due to time restrictions only fauna from 2 out of the 4 dredges were analysed). Anchor dredge survey: the dredge was deployed from the stern and left to ‘anchor’ for 1 minute before being retrieved, each sampled was sized and the residue was preserved in 4% buffered formalin in seawater. Once at the laboratory the fauna were identified to the lowest taxonomic level. At each anchor site sediment samples were also collected using a 0.1m2 Day grab.

Habitat Effects

The greatest abundance of scallops were found in sandy sediment, along with the richest communities and largest biomass of structural epifauna. These species can be removed by scallop dredging altering the community structure; a loss of this biogenic epifauna can have implications for juveniles. The problem being that the juveniles of some commercial species gain protection from predation by other species within these structured habitats.

Species and Community Effects

Within the sandy sediment samples the most important taxa were: Ophiurs albida, Turritella communis, Edwardsia sp., Photis longicaudata,/em> and Eunicidae. Within the gravelly sand sediment samples the most important taxa were: Owenia fusiformis, Eunicidae, Hydroides norvegica, Glycera sp. and Ophiura affinis. The results suggested that the emergent fauna such as soft corals, sponges and hydroids are vulnerable to removal fishing gear.
Effects to target species: Results from both the scallop dredge survey and anchor dredge survey indicated that the greatest abundance of species was found on sandy sediments, the lowest abundance of species was found on gravelly sand sediments.

Location

Fowe/Eddystone Ground (English Channel)

Fishing Types

Scallop dredge.

Reviewed by

Sewell et al 2007

Full Reference

Charreire, F., 1993. A report for Greenpeace on recent dolphin strandings along the French Atlantic coast , ,

Description

Both nets and trawls are involved in the incidental capture of dolphins however accurate estimates of by-catch cannot be made because of lack of relevant data. High opening pelagic trawls towed by pairs of boats and combined trammel-gill bottom nets tied together in a row about the continental shelf are perhaps the most likely cause of large dolphin by-catch.

Direct observations were made of a beam trawl, targeting the brown shrimp Crangon crangon passing over a Sabellaria spinulosa reef in the Wadden Sea, using a video camera attached to the trawl. A controlled 'before/after' experiment was carried out on a periodically exposed, 30 hectare, Sabellaria alveolata reef on the French Atlantic coast, using a 3 m research trawl equipped with ten rollers. The force exerted on the reef by the trawl was calculated as was the load bearing capacity of the reef.

Habitat Effects

The authors conclude that the type of 'light weight' beam trawls used by shrimp fishing vessels cannot cause damage to reef constructions.

Species and Community Effects

Following passage of the trawl over the reef, the authors did not notice any signs that the reef structures had been destroyed. Impressions left initially by direct contact from the trawl shoes had disappeared four to five days after the experiment due to rebuilding by the worms.

A survey of the fauna of an intertidal mud/gravel beach. Distributions of dominant species, which characterised assemblages were compared to similar studies carried out between 1958 and 1959.

Habitat Effects

Based on observations , the authors describe how bait digging brings gravel to the surface, leading to an increase in the gravel content of surface material. Holes dug by bait diggers tend to accumulate fine sediment, resulting in characteristic 'pock marks' surrounded by gravel. The authors note that these decrease in abundance as distance from public access points increases.

Species and Community Effects

Several differences were noted between the two surveys. There had been declines in numbers of the polychaetes Megalomma vesiculosum, Sabella pavonina and Arenicola marina There had also been a 'dramatic increase' in the abundance of Nereis virens. These changes were largely attributed by the authors to an increase in bait digging activity in the area.

Studied aimed to look at the effects that both otter trawls and scallop dredging have on benthic megafaunal communities in gravel habitats. Two cruises were carried out in 1994 looking to quantify the differences between disturbed and undisturbed sites. Six sites were surveyed with a side-scan sonar, 15 minute video transect transects (3 transects at each site), 1-3 stations were selected at each site for the benthic fauna sampling using a Naturalists dredge (3 replicate samples were collected at each station). In the laboratory the benthic samples were identified to their lowest taxonomic level. (2 survey sites in the US 17 & 18, 4 survey sites in Canada 10, 11, 13 & 20).

Habitat Effects

At three of the sites (10, 11 & 20) the seabed was comprised of sediment consisting of pebbles and cobbles with a high percentage cover of the tube-dwelling polychaete Filograna implexa, the presence of this species would indicate a lack of disturbance. The area also had a number of boulders which would become obstacles for both trawling and scallop dredging. However, on the side-scan sonar dark lines were visible indicating dredge tracks, this provided evidence of minor disturbance. Two sites (13 & 17) showed evidence of heavy disturbance as a result of dredging; the sediment was comprised of smooth pebbles and was almost devoid of encrusting organism like Filograna implexa that were abundant at undisturbed sites. The final site (18) appeared to be undisturbed, even though there were few obstacles that would cause problems for dredges and trawls. This site however may have been previously disturbed.

Species and Community Effects

When the undisturbed sites (10, 11, 18 & 20) were compared with the disturbed sites (13 & 17), they had a higher number of organisms, biomass was higher as was species richness and species diversity, evenness however was higher at the disturbed sites. At the disturbed sites the species dominating these areas were large, hard-shelled mollusks and scavenging crabs and echinoderms. In contrast to this the dominating species at the undisturbed sites were bryozoans, hydroids, worm tubes, which in turn provided a complex habitat for polychaetes, shrimps, small echinoderms, mussels and brittle stars. This shows a clear difference in the community structure between disturbed and undisturbed sites.

Photographic evaluation of the effects of scallop dredging on Georges Bank.

Habitat Effects

small differences in sediment type between dredged and undredged sites with dredged sites having a slightly higher frequency of small pebbles, and the undredged sites having slightly more larger pebbles and cobbles.

Species and Community Effects

Samples of benthic megafauna from disturbed and undisturbed sites showed that disturbed sites had lower density of organisms, biomass, and species diversity than undisturbed sites. Many of the species that were absent or less common in dredge sites were small, fragile polychaetes, shrimps and brittlestars. Most apparent difference was the lack of colonial, epifaunal taxa at the disturbed site. This study aimed to give a quantitative assessment of the impact using still photographs.

Comparison of deep sites showed that Filograna implexa had a high percentage cover at the undredged site and no epifauna and few animals visible at the dredged site. Significant effect between depth and dredging for both F. implexa and plant-like animals with effect on percentage cover greater at the deep sites. For plant-like animals the effect was higher at the shallow sites. Protula tubularia was significantly more abundant at undredged than dredged sites. There were no differences in the proportion of photographic sampling cells with bryozoans in them, but dredged sites had a significantly higher proportion of cells with abundant bryozoans than undredged sites. Spirorbis was more abundant at the deep sites and was in higher frequencies at the dredged sites than undredged sites. Most likely explanation is that the emergent epifauna at undredged sites concealed encrusting bryozoans and Spirorbis from view.

Depth had the greatest effect on the frequencies of non-colonial animals. Dredging had a lesser, but still significant effect on the frequencies of non-colonial species. Undredged sites had higher frequencies of almost all taxa except burrowing anemones, the earshell Sinum perspectivum and hermit crabs. Most of the non-colonial taxa seemed to be negatively affected by dredging but some seemed to profit from dredging. Burrowing anemones were more prevalent at dredged sites for example, perhaps because tentacles easily retracted to safety.

Results consistent with the hypothesis that gravel habitats are very sensitive to physical disturbance by bottom fishing and the primary impact is the removal of emergent epifaunal taxa.

Since the 1970s there have been seasonal closures in New England to protect the groundfish stocks, however these seasonal closures have had little impact. In December 1994 three large areas were closed year-round to all types of gear that could retain the groundfish. The studies assessed the changes to both scallop and groundfish stocks as a result of the year-round closures.

Habitat Effects

Species and Community Effects

Target species: from 1994 to 1998 the biomass of scallops within the closed area increased 14-fold and in July 1998 the total scallop biomass was 9 times denser and the harvestable biomass was 14 times denser in closed areas than the adjacent open areas.
Non-target species: closed areas led to a significant reduction in the fishing mortality of the depleted groundfish stocks. The closed areas provided the greatest protection to shallow-sedentary assemblages of fish species-mainly flounders and skates, less protection was provided to migratory species-Atlantic cod and haddock.

Dissolved oxygen levels were lower than for the control site from April to September.

Species and Community Effects

Community composition was similar to control sites, although species diversity, and densities were lower below oyster culture areas than at the control site. The author did not find anoxic conditions at the oystrer culture study site during the study and concludes that there were no conspicuous disturbances at the study site.

Assessment of the interaction between seabirds and warp cables in the Argentine hake trawl fishery, the efficiency of mitigation measured designed to reduce the mortality of seabirds in the warp cables was tested. Data was obtained from three different ice trawl vessels, during haulback activities seabirds associated to the vessel were identified to species level. Birds involved in incidental capture and interaction with warp cables (both non-fatal & fatal) were also recorded. During January & February 2006 a mitigation device was tested on board one of the vessels, again data was collected based on observations and interactions with warp cables.

Habitat Effects

Species and Community Effects

13 seabird species were recorded using the food that was made available by the fishery, the 2 most frequent were kelp gull and black-browned albatross. Interactions were recorded between both seabird nets and warp cables. Interactions with warp cables were the most frequent when birds were feeding on discards from the surface, but collisions also occurred when birds were flying low in the direction of the vessel. Contacts were recorded in 81.4% of the hauls, of which 81% were kelp gulls. During the 2004-2005 fishing season 53 individuals were killed as a result of interactions with fishing gear, of which 11.3% were cable related mortalities. As a result of the introduction of the mitigation measures there was a significant in the number of contacts between seabirds and fishing gear. In hauls with the mitigation measures contacts were reduced by 89% when compared to hauls without mitigation measures. No seabirds were killed as a result of contact with warp cables on the vessels where mitigation measures had been implemented.

Study aimed to assess and monitor the progress of the razor clam fishery in Gormanstown that started in 1997. During the year long study (1998/99) 25 samples were collected, 10 were obtained by boarding the commercial vessels and collecting a fraction of the dredge contents and 15 were provider by the fishermen themselves. Once in the laboratory the dredge contents was identified and weighed, the clams that were intact were measured and any breakages were recorded.

Habitat Effects

Species and Community Effects

The dredge samples contained a variety of animal species and sediment; the sediment consisted of fine sand to coarse shelly material. The species that were occasionally found in large numbers were Lanice conchilega, Pharus legumen and Donax vittatus the abundance of these species was often associated with very small razor clams (<6 cm). Hydraulic dredging was shown to have short term effects; however the physical consequences were not visible after 40 days. It was long-lived species that were likely to take much longer to recover.

Three year study into the effects of otter trawling on a sandy-bottom ecosystem of the Grand Banks. Sediment samples, acoustic measurements and video surveys undertaken.

Habitat Effects

Statistical analysis of seven size fractions gave no evidence that trawling had any immediate effect on sediment grain size. Sidescan sonar showed the persistence of door tracks was variable from several months to a year. Acoustic data suggest that repeated trawling did not affect sediment texture but increased surface relief or roughness. Small-scale biogenic sediment structure down to 4.5cm also changed. Video surveys showed clear differences in the appearance of the seabed. After trawling hummocks were removed or less pronounced, organic floc was either absent or less abundant and mottled appearance of the seabed less pronounced. Sediment grain size data suggest that there may be natural inter-annual changes that are more pronounced than those caused by the experimental trawling.

Species and Community Effects

Video imagery showed organisms and shell has organised into linear features in the trawled areas. At times high concentrations of Strongylocentrotus pallidus were visible and seemed to be scavenging on dead snow crabs. Biological effects have still to be examined.

Study of effects of trawling with an Engel 145 otter trawl with rockhopper gear and a door spread of 60 m. Trawl catch and remaining epibenthos were analysed. Three experimental sites were trawled 12 times each, with a period of five days once a year for three years. Comparison were made between trawled and untrawled sites.

Habitat Effects

Species and Community Effects

Studies revealed an influx of scavenging snow crabs following trawls. The biomass of benthic organisms was 24 percent higher in untrawled sites compared to trawled site. The homogeneity of the sampled macro-invertebrate community was lower in trawled than untrawled sites.The experiment indicated that otter trawling on a sandy bottom ecosystem can produce detectable changes on both benthic habitat and communities, in particular a significant reduction in the biomass of large epibenthic fauna.

A three year study on a deepwater (120 - 146 m), sandy bottom ecosystem that had not been trawled for 12 years. Two 13 km long corridors were trawled 12 times a year within a five day period, using an 'Engel 145' otter trawl.

Habitat Effects

Species and Community Effects

Most prominent feature of the data was a natural decline in the total number of species, the total abundance and biomass of selected species between 1993 and 1995. In 1994 however, the abundance of 13 species, the biomass of 11 species (mostly polychaetes) and the total abundance per grab reduced significantly in trawled areas compared to untrawled areas. The authors found little evidence of trawling effects.

Dredging tracks were formed and stayed for several months. Sediment was physically removed and dominant sediment type was altered to make it unsuitable for the settlement of mussels Mytilus edulis.

Species and Community Effects

No significant effect of fishing was found for densities of Hydrobia ulvae or 0 - 1 year class Cerastoderma edule. No Mytilus edulis were found in heavily trawled areas and this was considered to be a direct result of the physical effects of trawling. There was a signifiant negative effect of fishing on young (2000 year class) Macoma balthica, but the effects on older individuals could not be tested.

Effects of mobile fishing gear at three sites on a variety of bottom types in the Gulf of Maine were investigated.

Habitat Effects

Habitat complexity was reduced by direct removal of biogenic and sedimentary structures and the organisms that create structure eg. reduction of an extensive sponge community to the occasional small colony on large boulders, absence of previously widely distributed ascidian, reduced density of shrimp, dispersal of shell deposits by mobile gear.

Species and Community Effects

Authors discuss how this reduction in complexity may lead to increased predation on juveniles of harvested species and ultimately recruitment to harvestable stock especially in the northeast USA, where fish assemblages are part of a system where predation mortality on postlarval and juvenile fishes has a major effect on year-class strength.

Survey of lost gill net over a three year period using submersible. Known ghost net sites at depths between 30m and 127m on a variety of seabed types, surveyed quantitatively by transects. 700m long ghost net on Stellwagen Bank in a boulder field grading to silt-clay substrate was visited on two occasions.

Habitat Effects

Species and Community Effects

Species caught include dogfish, bluefish, lobster, spider crab and edible crab. Hagfish were often seen preying on the dogfish and bluefish. A 470m long ghost net surveyed for two consecutive years had dogfish as the most predominate vertebrate catch. Cancer crabs were the most common invertebrate catch. Codfish were not seen in the ghost gillnet, nor were there identifiable remains of cod at the base of the net.

Experimental tow using one 3 m wide rapido trawl over a relatively undisturbed sandy-bottomed scallop bed. Authors used underwater video before, during and one and 15 hours after trawling and catch analysis to study the effects of the trawl on the benthos.

Habitat Effects

3 m wide tracks were left, following the trawl. Sediment was flattened, with no worm tubes or burrows that had been there previously. Tracks were littered with animal and shell fragments.

Species and Community Effects

Mobile scavenging organisms, particularly spider crabs, hermit crabs and some fish species increased in abundance in trawled areas. Significant decrease in abundance of and obvious damage to the fan shell Atrina fragilis. Coralline rhodoliths were smashed and dispaced or buried by the trawl. Large numbers of soft bodied tunicates were killed by the passage of the trawl and/or caught as bycatch. Trawl teeth speared soft bodied invertebrates and large, hard-shelled bivalves. Damage to benthos was limited to organisms living within the top 2 cm of sediment. Large, fragile organisms, generally sustained the highest levels of damage when caught by the trawl, whilst smaller, hard-shelled organisms were fatally damaged only in low proportions.

A study, examining the impacts of otter-trawling for demersal fish and shrimps at a known fishing site in the Medirerranean. The benthic macrofauna and sediment chemistry was studied at two sites within the trawl lane and two sites outside the known fishing area. Towed video surveys and beam-trawl sampling were used to study seabed conditions and macrofauna.

Habitat Effects

Scrape marks and a 'general flattening of the microtopography' were observed in trawled areas and the resuspension of sediment was apparent.

Species and Community Effects

Significantly lower numbers of epifauna were found at unfished sites, particularly large echinoderms. Species number, abundance and biomass were all significantly lower during the trawling season in the trawl lane and there were significant differences in sediment characteristics between fished and unfished sites. The study indicated that the four month closed season currently in place did not allow time for recovery of these factors to pre-trawl levels.

Using a bio-energetics model (described by Phillips et al. 1999) to estimate the amount of prey consumed by the great skuas at Hermaness, Shetland. Including the composition of the prey consumed and how changing input parameters affects the prey that the great skua consumes.

Habitat Effects

Species and Community Effects

The total energy required by the great skua colony increased from 1999 (491.5x106 kJ) to 2001 (546.6x106 kJ) by 11.2%. Most of the prey consumed by great skuas was fish followed by seabirds. In 1999, 80,000 kg of fish were consumed and 7,610 kg of seabirds; in 2001 the amount of fish consumed increase to 90,000 kg, despite predicted declines in discards, the number of seabirds consumed declined but only by 150 kg. During the 1999 and 2001 breeding seasons it was estimated that the great skuas consumed more than 12,500 and 13,000 birds respectively. The three most commonly consumed bird species were auks, northern fulmars and black-legged kittiwakes.
Changing the input parameters had a profound affect on the diet of the great skua, a decrease of 50% in the number of fish consumed resulted in an increase of more than 50% of bird species consumed. This current level appears to be unsustainable for prey populations indicating the importance that fishery discards have in the system in determining seabird predation.

Study examining the effects of Skua predation on smaller seabirds, particularly regarding reproduction and survival. The parameters were compared with individuals feeding only on fish.

Habitat Effects

Species and Community Effects

Specialist bird predators spent less time foraging than skuas feeding predominantly on fish. Results of radio-telemetry indicated that bird-specialist skuas have smaller home ranges than other birds. In a comparison of reproductive performance, specialist bird predators consistently hatched earlier among years. They also showed larger clutch volumes and improved chick condition, but these were subject to annual variations. Hatching success and fledging success for specialist bird predators and specialist fish predators were similar. Specialist bird predators showed similar annual survival compared with fish-feeders over the same period. Specializing as a bird predator may be limited to the best birds in the population, but their poorer than predicted breeding success reveals the need for further study into the relationship between diet and reproductive success in this species.

Paper tests priori predictions taken from the literature of changes in population, taxonomic and functional groups, as well as looking at community-level characteristics with changes of fishing pressure. A high resolution side scan sonar was used to survey the seafloor and identify sites with differing levels of fishing activity and environmental conditions. An ROV was used to conduct video transects of the seafloor at each site; this enabled the density of large epifauna to be estimated. At a central area of each site macrofauna was sampled, at muddy sites with was done using a grab and at sandy sites using a suction dredge. 15 core samples were also randomly taken at each site.

Habitat Effects

Side scan sonar indicated a number of scallop dredge marks in the sediment. One of the most important ecological effects is related to changes in habitat complexity, the removal of organisms that add complexity to benthic habitats can be very destructive as can the homogenization of sediment characteristics by the physical action of both trawls and dredges. Both can cause a reduction in spatial heterogeneity over a range of ecologically important scales. From a theoretical point heterogeneity is an important component of ecological systems which can have implications fro the maintenance of biodiversity and stability at all levels (population, community and ecosystem level).

Species and Community Effects

Results from the video data indicated that for each site as a whole the density of epifauna produced the results that were predicted, as fishing pressure decreased the density of large epifauna increased. The grab and suction dredge data indicated that density of deposit feeders, total number of individuals and epifauna were all influenced by fishing pressure, although not all were significantly influenced. In each case as fishing pressure decreased the density of deposit feeders, total number of individuals and epifauna increased. The predictions tested have important consequences for changes in both sediment structure and function of benthic communities.

Studied aimed to compare the benthic fauna found in areas that had been exposed to different intensities of bottom-fishing (high or low) over a 10 year period. Data on fishing effort was obtained from ? of the dredging fleet via log books. Ten sites were selected (5 high intensity and 5 low intensity), at each site the following data was collected, 3 day grabs for organic content and sediment particle-size analysis, 3 infaunal samples using an anchor dredge (deployed 1 minute on seabed) and 3 epifaunal samples collected using a 2-m wide beam trawl (towed for 5 minutes). In the laboratory faunal samples were identified to species level where possible and biomass and total number of species was quantified.

Habitat Effects

The study showed that the disturbance as a result of scallop dredging had led to changes in the community structure of both benthic habitats. The removal of organisms that contribute to the complexity of the habitat may result in the degradation of the habitat to a point where it becomes unsuitable for associated species as well as the target species.

Species and Community Effects

Abundance and biomass data for the epifaunal and infaunal samples was plotted on K-dominance curves. The results for the low intensity fishing areas showed that the biomass curve was above the abundance curve, this indicated that the community was dominated by a small number of large-bodied organisms. For the high intensity fishing areas the two curves converge, this indicates an increase in physical stress; the community was dominated by a high number of small-bodied organisms. Within the high intensity fishing areas the biomass of brittlestars Ophiura albida and Ophiocomina nigra was highest and the biomass of soft corals Alcyonium digitatum, the large sea urchin Echinus esculensis, the bivalve Glycymeris glycymeris and the gastropod Buccinum undatum were lowest. This change in structure indicated that as a result of repeated dredging the large-bodied organisms have been removed and replaced with small-bodied organisms that are less susceptible to disturbance.

Data on the benthic community in the Irish Sea was collected by N.S. Jones between 1938 & 1950. The aim of this study was to determine whether over the last 40-60 years the benthic communities had changed and if these changes could have been caused as a result of scallop dredging. To achieve this seven of Jones’ sites were re-sampled.

Habitat Effects

Results from the effects of scallop dredging on sediment indicated that during the 40-60 year study period the sediment at four of the sites (for which there was historical data) became finer and possible at another two sites if estimates of the historical data are correct. The decrease in sediment size made no difference whether the seabed was stony, sandy or gravely initially.

Species and Community Effects

Community composition was shown to change at all seven sites, but to varying degrees, the amount of change was related to how long the site had been dredged for as opposed to the intensity of the dredging. Species that were considered to be robust, mobile and scavenging had increased in abundance, where as in comparison organisms that were fragile, slow-moving or sessile had decreased in abundance.

Paper looks at the extent to which fishery discards and offal influence the food choice of large gulls resting on Helgoland, their resting numbers and their body mass and condition. The period of heavy fishing (December 1997 to March 1998) was compared to times when fishing was suspended during this time frame (Christmas to New Year 1997 and February 1998).
Trawl activity: all fishing trawlers that were in range of sight of Helgoland from December 3rd 1997 to March 30th 1998 were counted.
Diets: 233 fresh pelts were collected from the piers where the gulls usually rest during the night (127 were collected during the periods with no fisheries and 106 were collected during the periods with fisheries). The contents of the pellets were identified to species level where possible.
Body condition: the body mass of the gulls in relation to head and bill length was used as indicator of body condition. Between December 7th 1997 and February 26th 1998 gulls were caught on their roost piers during the night. Each gull was weighed and aged (birds < 3yrs said to be immature, birds > 3 yrs said to be adult).
Gull counts: counts were carried out in December 1997 and January 1998.

Habitat Effects

Species and Community Effects

The results indicated a noticeable different in the diets of the gulls between times when fishing took place and times when fishing did not occur. During times of high intensity fishing (first half of December 1997), more than 80% of the pellets contain discards, indicating that gulls feed primarily on discards when available (the dominant species found was cod). During the periods when fishing activities did not occur, the gulls fed on food items within the rocky intertidal zone, terrestrial food and garbage (some pellets however still contained discard remains – gulls may have foraged outside of the study area).
Fishing activity was also shown to affect the body condition of the gulls. The mean body mass of the adult herring gulls decreased by 13% during times of no fishing activity and the mean body mass of the adult great black-backed gulls decreased by 24% during times of no fishing activity. Results indicated a poorer body condition for adult gulls at times with no fishing activity, for immature gulls there was no significant difference in body condition between times of fishing activity and no fishing activity.
At the start of the study (December 1997) over 3000 herring and black-backed gulls were resident on the island; however this number dropped drastically to around 800 gulls when the fishing activities stopped at the end of the year. In January the fishing activities resumed and the number of gulls increased to nearly 5000, but fell again during the closed period in February. Although fishing activities did not really increase during March the number of herring gulls did, possible due to the presence a new natural food source.

The paper describes condition of a dead otter found on the beach near Scarista on the Isle of Harris.

Habitat Effects

Species and Community Effects

The otter was emaciated and the cause of death strands of monofilament nylon which had become embedded into the flesh around the neck. It was a small section of fishing net (square aperture approximately 50mm). It seems likely that the otter was entangled at an early age (3-5 months) and as it grew the nylon became enclosed in tissues of the neck. Unknown how many are lost in this way and whether it is large enough to be a conservation problem and one of animal welfare. Needs monitoring. This case shows that even a small section of discarded net can be lethal therefore the solution is difficult.

Review of study investigating disturbance by scallop dredging from large (fishing grounds) to small-scale (experimental plots) around the Isle of Man

Habitat Effects

Species and Community Effects

Dredging disturbs and may be a factor in structuring benthic communities on gravelly sea bed. Community composition is related to the intensity of commercial dredging effort and effects may differ from that of bottom fishing on other soft sediments due to extreme patchiness of animal distribution, greater abundance of epifauna and to the combined effect of the heavy, toothed scallop gear and the stones caught in the dredges.

The paper reviews the results of a large study, examining the ecological effects of disturbance by scallop dredging at both large and small scales on gravelly seabed communities.

Habitat Effects

Unfished areas were found to be less homogeneous than dredged areas, supporting more diverse species assemblages. Following the onset of the annual closed season

Species and Community Effects

Large scale: The composition of species assemblage differed greatly between dredged and un-dredged sites and this was thought to be a direct result of dredging activity. However species diversity and dominance of epifaunal assemblages did not differ greatly between dredged and undredged sites. Dredge disturbance in a previously closed area: Infaunal communities in experimentally dredged sites, within an area that had been closed to fishing for nine years quickly altered and became very similar to survey sites in heavily dredged areas.

The role of the sandeel fishery was assessed in relation to the decline in the population of the black-legged kittiwakes. Demographic data collected from 1986 to 2002 was examined for changes and correlations between population parameters, local sandeel fishery and environmental factors. The results were incorporated into a deterministic and stochastic matrix population model.

Habitat Effects

Species and Community Effects

From 1969 to 1990 there was a continuing increase in the number of completed kittiwake nests (4801 nests to 8129 nests), however numbers declined to 3666 in 2002. Breeding success has varied considerably from 1.24 fledged chick per nest in 1986 to 0.02 in 1998. It was during the 1986-1989 season that breeding success was high (1.07), however, the following 10 years (1990-1999) was when the Wee Bankie sandeel fishery was active and breeding success fell to a mean of 0.30. The fishery closed in 2000 leading to a small recovery from 2000-2002 (mean = 0.68).
The results indicate that kittiwakes are almost completely dependant on sandeels during the breeding season; this therefore means that any activities that reduce the abundance and availability of sandeels will likely have a negative effect on kittiwakes and lead to declines in the population.

Authors studied the relationship between macrobenthic species diversity and habitat complexity at 10 spatially separate sites. Experiments were carried out in a 10 - 20m deep large embayment, composed mainly of simple, soft-sediment habitats, varying in sediment and structure.

Habitat Effects

Species and Community Effects

The findings of the report strongly suggest that biodiversity is directly related to habitat complexity and that human activities (particularly trawling and dredging) that remove epifauna and lead to habitat homogenisation will reduce biodiversity in soft bottomed habitats.

Laboratory experiments to see whether non-lethal burial or exposure on the sediment surface could alter the normal living depth of Mya arenaria in sand and mud.

Habitat Effects

Species and Community Effects

After 2 weeks those buried under 1-15cm of medium fine sand were buried deeper than controls whereas clams exposed on the sand surface (and had subsequently reburrowed) were able to re-establish their normal living depths. Clams under 1-15cm of mud attained their normal living depth within two weeks but exposed clams reburrowed to abnormally shallow depths. The increased likelihood of predation at shallow sediment depths was compounded by the 60 percent lower reburrowing speed of exposed clams in mud when compared to sand.

Conclusions were that negative impacts of clam digging on M. arenaria are not limited to removal of market-size clams and shell breakage of remaining ones. Exposure of prerecruits and depositions of tailings on clams adjacent to harvest sites may increase susceptibility of unharvested clams to predation, dessication or freezing. The effects depend on different substrate types. Mortality will be greater on clam flats having a mud substrate than of medium-fine sand. Management practice should reflect these differences. On sandflats there would be little to be gained from breaking up the clumps of soil turned over since tailing burial will probably not result in mortality. In muddy areas, reducing tailing piles is likely to enhance survival of both buried and exposed clams.

Study to test a novel shear vane for the analysis of Hydraulic dredge tracks. Used to analyse sediment characteristics following dredging activity.
Test study undertaken in on a razor clam (Ensis) bed in a sheltered harbour area, with low tidal flow, in water depth - 4-6m below Chart datum. 2 sites subjected to a single tow using a UMBSM Hydraulic suction dredge. 2 undredged control sites. Diver measurements and core analysis used to assess recovery in addition to shear vane analysis.

Habitat Effects

After 100 days, track depth decreased from 13.9 cm to 2.9cm and width increased from 100 – 110 cm due to bank erosion. Vertically homogenised sediment 9reduced stratification for depths exceeding 20cm. Impact remained apparent for more than 100 days, probably due to the sheltered (tidal flow did not exceed 0.5ms –1) nature of the study site. This was compared to a recovery time of only 40 days from a previous study in a more exposed area in the Sound of Ronay, Scotland (tidal flow exeded 1.5ms-1).

Species and Community Effects

Authors suggest that repeated passes by Hydraulic dredges in sheltered areas could have a serious impact on biological community structure and the persistence and ultimate consequence of these activities should be carefully assessed.

Location

Lamlash Harbour on the Isle of
Arran, Scotland

Fishing Types

Suction dredge.

Reviewed by

Sewell et al 2007

Full Reference

Southern Science., 1992. An experimental study on the impact of clam dredging on soft sediment macro invertebrates Report to English Nature.

Description

Treatment and control type dredging experiment, 2 passes of a modified oyster dredge.

Habitat Effects

Sediment removed to a depth of between 15-20cm by dredging and gravel fraction reduced. Sediments may become more anoxic after dredging. Dredge tracks most likely to be filled with fine sediment in low energy conditions therefore discrete habitat variation will be created. Resuspended sediment may have serious survival implications for species unable to deal with heavy suspended sediment loads.

Species and Community Effects

Due to the deep penetration of the dredge all fauna, with the exception of bivalves (eg Abra tenuis, Cerastoderma edule and Mya arenaria) were removed completely in the short term. It is likely that these organisms were dislodged and then redeposited by the dredge or that they migrated or were passively dispersed into the area from adjacent undredged areas. Annelids were most badly affected by the dredge with the exception of Tubificoides benedeni and a phyllodocid. Abundance of bivalves was also greatly reduced but some found in some dredged samples (small specimens thought to have been disturbed by the dredge and re-deposited afterwards).

No clear recovery of fauna evident over the 8 day period of study but opportunistic polychaetes (eg Capitella capitata and Tubificoides benedeni) likely to be early colonisers of disturbed mudflats along with the surviving bivalves. Authors suggest these will be followed by active polychaete species eg Eteone longa and more stable habitat species such as Cirriformia tentaculata. Continual disturbance will not favour stable habitat species, high biomass communities may occur but are unlikely to contain individuals of high biomass which may be exploited as a food source by birds.

Two experimental regimes. Experiment 1: Effects of a single dredging activity. Experiment 2 at Lavan Sands 80 sampling stations over an area of 400x300m used to assess the effects of a 3 month licensed commercial dredging operation using pre and post dredging data.

Habitat Effects

Experiment 1 - Dredging had no significant impact on the measured sediment characteristics due to the small percentage of fine material and the high degree of sorting.Experiment 2 - No severe erosion of sediments occurred.

Experiment 2 - Impacts appear to be small and for the most part not statistically significant. Significant decrease in the population of tube dwelling polychaete Pygospio elegans whose tubes may be destroyed by dredging. Lanice conchilega has tough tubes apparently not greatly affected by the dredging operation. Also they can retract into tubes below the maximum depth disturbed by the dredge and can regrow head tentacles. Numbers of Cerastoderma edule and Macoma balthica reduced significantly resulting in a significant reduction in the total macrofaunal biomass (these molluscs contribute to about 70 percent of the biomass wet weight).

Author concludes hydraulic cockle dredging unlikely to have a significant impact on non-target infaunal species at the site as the sediments are moderately mobile with a low silt content.

Sampling 0, 30 and 60 mins after fishing. Immediately after mussel dredging suspended particulate material increased significantly but 30 mins after the differences had decreased and, after 60 mins, had returned to the start level. Oxygen decreased significantly after mussel dredging and average ammonia content increased but large horizontal variation in the ammonia content prevented detailed interpretation of these increases. Changes in other nutrients were small. Changes in particulate matter and nutrients were also observed at some stations following low wind. Particulate matter and total phosphorus were markedly higher on windy days.

Most dredging and trawling in the Limfjord takes place in summer when there is little wind, nutrients and oxygen consumption are low and temperature high. During these periods trawling and particularly dredging reduce the water quality by increasing internal nutrient loads, oxygen consumption and possibly phytoplankton primary production. Immediate increase in particulate matter, oxygen consumption and increase in nutrients particularly ammonia and silicate were a further effect of the fishing activities. Physical effects were scraping and pressure of gear the magnitude depending on depth of penetration, frequency of fishing and structure of sediment.

Species and Community Effects

Trawling and dredging can be expected to cause a number of direct and indirect changes in the ecosystem - direct changes in fished populations and the benthos, but also changes in the nutrient level and oxygen budget in the water column. Phytoplankton primary production may increase if nutrients are the controlling factor. During summer when nutrients are generally low in the fjord mixing of sediments will have important consequences for the nutrient regime. It caused the deterioration of the water quality by increasing oxygen consumption and phytoplankton primary production. It was difficult to demarcate trawling and dredging effects versus wind induced effects at this site.

Experimental work in situ and in laboratory to evaluate the importance of the upwelling of sediment during dredging and, in particular, the amount of sediment particles, nutrients and oxygen consuming substances released during dredging as these factors can effect macrophyte and phytoplankton growth as well as affecting fish and bivalves.

Habitat Effects

Preliminary results suggest a minimum flux of 2km2, corresponding to about 0.9cm penetration of the gear. The release of particles, nutrients and oxygen-consuming substances seems to have little effect on the overall environmental conditions in the fjord. Where 10-15 boats dredge for several days, authors note that this will alter the local concentrations of nutrients and suspended matter directly, but the effect would probably only be visible or significant, during the dredging operations. Total annual release of suspended particles shown to be relatively unimportant compared with total annual wind-induced resuspension and release of nutrients compared to load from land.

Species and Community Effects

the effects are probably much more severe on the ecosystem by changing the bottom flora and fauna which may in turn affect water quality. If natural bottom community cannot be established the areas will be characterised by low biodiversity and by opportunistic species dominated by young individuals of small sizes. Overall environmental effects of this disturbance in Limfjorden is not fully understood.

Experimental beam trawl over a 4x2km area, at a depth between 26 and 34m. A commercial beam trawl, weighing 3.5 tonne fitted with a chain matrix and 8cm diamond mesh cod-end was used. Waylines were fished either 10 or 20 times to adequately disturb trawl area.

Habitat Effects

Physical characteristics of the surface sediment were altered by the passage of the beam trawl but effects varied in different parts of the experimental area. Surface roughness of the relatively uniform, stable, flat areas were not altered by trawling but lowered in fished sites in the SE sector which was characterised by sand waves and some ripples. In the latter case the surface ripples were flattened but the megaripples were unaffected. Passage of the chain matrix may have caused sediment to become unconsolidated as shell and gravel currents. Conclusions were that particle size distribution was not affected and observed changes may only be in the superficial layers of the sediments. Newly exposed shell and gravel material would provide surfaces for recolonisation and settlement, epizoites on surfaces which were overturned would be smothered.

Species and Community Effects

Beam trawling altered the benthic community structure in the uniform, stable, flat areas having a measurable deleterious effect on the number, abundance and diversity of taxa. Of the top 20 most common taxa, abundance of 19 were lowered at fished sites, nine of which were statistically significant. Fragile infaunal species which live on or within the surface sediments (bivalves, holothurians, gastropods) were particularly vulnerable to damage or disturbance. The abundance of sedentary and slow-moving animals organisms was significantly lowered. Some animals were fatally injured or crushed, others only damaged (eg cropping of Mya siphons). Tissues of animals damaged by beam trawling rapidly attract scavengers. Analysis of diet indicated they were feeding on the damaged animals, most notably Ampelisca spp. There were no detectable differences in the diversity and abundance of taxa in the areas characterised by mobile sediments and subject to frequent natural disturbance.

Study examining the potential impact of windfarms on common scoter populations in Liverpool Bay

Habitat Effects

Species and Community Effects

Relevant findings from the paper include. The common scoter is displaced by shipping activity and resting flocks are often put to flight following disturbance. Male and female birds arrive in the bay at different times of the year, meaning that activities at different times may affect different sectors of the population. Direct observations indicate that most fishing activity in the area takes place in water depths exceeding 20m, which is beyond the depth at which the scoter is likely to forage and therefore does not interfere with scoters except on their inbound and outbound journeys.

Large scale study was undertaken using aerial observations to assess the spatial distribution of the Common Scoter in relation to prey abundance and environmental and anthropogenic variables that affect the efficiency of foraging. To assess prey types two 0.1m2 grab samples were taken and the contents sieved over 1mm mesh (see further notes).

Habitat Effects

Species and Community Effects

Disturbance appears to affect common scoter distribution, as shipping activity increased the number of birds observed declined. Only 2.65% of common scoter were observed during overflight observation in areas of heavy shipping activity, whereas 18% were observed in areas of intermediate shipping activity.
The study concludes that Common Scoter distribution is strongly influenced by the distribution and quantity of prey; these factors however are influenced by a combination of physical parameters.

Experimental dredging of sandflats with mechanical cockle dredge. Two distinct sites sampled. Site A: Poorly sorted fine sand with small pools and Arenicola marina casts with some algal growth. Site B: Well sorted fairly coarse sand, surface sediment well drained and rippled as a result of wave activity.

Habitat Effects

Dredge track visible after 6 months at Site A (stable sediments). No alteration in sediment parameters by dredging at Site B (mobile sediments).

Species and Community Effects

Effects of dredging on biota apparent at Site A after 3 months may be attributed to destruction of seabed algal covering, destruction of permanent tube dwellings, mortality of eggs/broods, interference with predator prey relationships or changes in sediment characteristic. Seasonal perturbation e.g. produced by winter storms produce community changes of greater magnitude than those caused by dredging in unstable high energy environments such as Site B.

Site A (stable sediments): Decreased number of Pygospio elegan no recovery to pre-dredging numbers by six months. Disappearance of Scoloplos armiger from some dredged plots. Distribution of Nephtys hombergii disturbed by dredging recovery after six months. Large decline in numbers of Hydrobia ulvae, statistical difference between dredged sites and control sites up to six months post-dredging. Cerastoderma edule numbers reduced by dredging, significant reduction in numbers compared with the control still apparent up to six months post-dredging.

Site B (mobile sediments): Populations of Bathyporeia pilosa exhibit greater fluctuations in numbers of individuals post-dredging. Initial reduction in the population densities of Hydrobia ulvae, Pygospio elegans, Cerastoderma edule, Nematoda spp. and Psammodrilaida after dredging followed by rapid recovery (no difference between control and experimental plots after 14 days). Increase numbers of Nematode attributable to dredging.

Two year study of macrofaunal succession and sedimentary biogeochemical parameters of seabed after intensive fish farming discontinued at 3 sites.

Habitat Effects

Species and Community Effects

All sites had low numbers of taxa at the beginning of the survey which increased in the two years but one site remained impoverished. The increase showed large fluctuations in one case which the authors attribute to a secondary input of organic material to the site which was considered to have set back recovery by at least 6 months. This points to the sensitivity of recovering sediments to additional stress. Improvements in terms of increased numbers of species and increased redox potential were recorded together with decreases in organic carbon, nitrogen and pore-water ammonia.

Field experiment of impact of fishing for razor clams Ensis sp. by hydraulic dredging on the associated infaunal community, 7m depth.

Habitat Effects

Species and Community Effects

Infaunal samples were examined at 1 and 40 days from fished and unfished plots. There were differences in mean number of species and individuals for control and fished sites 1 and 40 days later but only total numbers of individuals significantly lower. After 40 days no detectable difference. No statistically significant differences in the 10 most abundant species Bathyporeia elegans, Siphonoecetes kroyeranus, Exogene hebes, Spio filicornis, Corophium crassicorne, Streptosyllis websteri, Cochlodesma praetenue, Nephtys cirrosa, Megalorupus agilis and Perioculodes longimanus between treatments after either 1 or 40 days.

Suction dredging for Esnis had profound immediate effects on benthic community structure with consistent reductions in the numbers of many macrofaunal species and the target species. However, despite the relatively large scale nature of the disturbance, these effects appear to persist for only a short period. After 40 days no detectable difference - visually or from macrobenthic community analysis, effects on long-lived bivalves could however be more serious, and action of the dredge is violent enough to often crack shells of adult Arctica islandica. Larger polychaetes and crustaceans are also often retained on the conveyer, crushed in the mechanism or fall off the end to fall at random on the seabed. No estimate was made of survivorship of these individuals but many scavenging hermit crabs were active immediately after dredging. Migration and passive translocation play a part in returning the abundance of species to pre-impact levels. Authors suggest that local population reductions due to dredging are only likely to persist in a habitat if one of two conditions are met: (a) macrobenthic populations themselves, or the sediments in which they live, are immobile or (b) the affected area is large relative to the remainder of the habitat such that dilution effect cannot occur. For most habitats where Ensis could be fished authors believe that neither of these conditions likely to hold. Current technology restricts this type of fishing to approximately 7m therefore likely to be strongly influenced by wind and tide-induced currents in these areas. Sediments are probably mobile and effects will be diluted rapidly. However they note there is little knowledge of the relative importance of the various processes which contribute to animal movement and whether certain habitats may be more susceptible to persistent damage than others. At most sites the authors believe there will be adequate areas to dilute effects but prior examination of potential fishery sites is warranted.

Target species removed in great numbers, long-lived bivalve species often damaged or killed and smaller-bodied infauna either displaced or killed. With the exception of large bivalves, it would appear that effects on macrofaunal community in general are not locally persistent, although in calmer seasons effects may persist for longer than observed here. Another consideration is that if Ensis and other large bivalves play an important role in structure of benthic communities, their removal would result in cascading effects over long time scales. But in the high levels of sediment mobility at the study site, this hypothesis was considered unlikely.

Study of the effects of extensive and repeated trawl disturbance over 18 months followed by 18 months recovery in an area which has been closed to fishing for over 25 years. Reference and treatment areas sampled.

Habitat Effects

The relative differences in roughness between the treatment and reference areas increased during the disturbance programme and declined during the recovery period. The sediment in both areas was poorly sorted fine silt and trawling disturbance did not appear to have any effect on the sediment characteristics but trenches were left in the sediment by the trawl doors. Differences in organic carbon levels were not thought to be ecologically significant. More than 18 months was required before the physical characteristics of the sites became indistinguishable.

Species and Community Effects

Changes over time in abundance of individuals occurred at both sites but a treatment effect was also observed. Species numbers were greater at the treatment site after 16 months and remained so throughout the monitored recovery period. Numbers of some individuals were also significantly greater at the treatment site after 10 months disturbance (eg. Chaetozone setosa and Caulleriella zetlandica) only returning to similar numbers after 18 months recovery. Others declined in density (Scolopolos armiger and Nephtys cirrosa). There were no detectable effects on infaunal biomass. Community effects extended beyond the 18 month recovery period studied. Such recovery times suggest that even fishing during a restricted period of the year may be sufficient to maintain a community in an altered state.

Study aimed to assess how effective an underwater playback system (UPS) was at deterring eiders from feeding on mussel farms in Scotland. Two survey sites were assessed: Loch Striven and Loch Creran (before the surveys took place observation tents were set up at least 100m from the mussel farms to allow the birds to become used to their presence). In Loch Striven the trials were broken down into blocks of 6/7 day observation periods, the first period before the UPS was switched on, the second with the UPS switched on constantly during daylight hours and the third with the UPS switched off. In Loch Creran the observations before and after the UPS use were the same, the difference being that the UPS was switched on for 21 days constantly during daylight hours to determine long-term effects (the 21days were broken down into 3 lots of 7 day blocks). The UPS unit was secured to the central raft for raft based farms or to a moored platform in the centre of long-line farms (speaker was held 3 metres below the surface of the water). During the trails boat-chasing was carried out by the farmers as usual.

Habitat Effects

Species and Community Effects

As a result of the UPS there was a 50-80% reduction in the number of eiders feeding on the mussel beds, the control showed no reduction in numbers (playback was of an unassociated noise). The return time of birds to the beds after being chased away also increased. Therefore if the UPS reduces the number of eiders feeding on the mussels beds in a similar way as the presence of workers does, then the UPS maybe a useful deterrent when workers are not present on the mussel beds.

Paper looks at the importance of the marine bivalve Limaria hians as a reef-forming species in the United Kingdom and the key architectural role this species plays with benthic communities. Observations of Limaria hians reefs were made in Loch Fyne, this involved more than 80 dives (30-60 minutes long). During the survey period a commercial scallop dredge (with 6 Newhaven dredges per side) was towed though a section of the reef (an area at Creag Gobhainn). Dives 3 hours after dredging and over the following 3 days allowed the effects of the dredging to be recorded. Due to the nature of this habitat suggestions for the future were also made.

Habitat Effects

Three hours after dredging it was clear the reef had been ripped apart, with vast amounts being removed along the dredges path (this was from a single pass of the gear). Left on the dredge track were damaged Limaria hians, which attracted a large number of scavengers. During dives over the following 3 days the flesh from the file shells were consumed by a number of species within 24 hours including juvenile cod, edible whelks, hermit crabs, dogfish, dragonets, swimming crabs and brittlestars.

Species and Community Effects

Location

Loche Fyne

Fishing Types

Scallop dredge.

Reviewed by

Sewell et al 2007

Full Reference

Brown, B. & Wilson, W.H., 1997. The role of commercial digging of mudflats as an agent for change of infaunal intertidal populations. Journal of Experimental Marine Biology and Ecology, 218, 49-61

Description

Study examining the effects of digging for bivalves and worms using a four tined hoe on an intertidal mudflat. Two 1m2 plots were dug and one 1m2 plot remained undug as a control plot. The two dug plots were given different digging treatments over the 2.5 month treatment period. The first was dug twice a month (low frequency) and the second was dug twice a week (high density).

Habitat Effects

Species and Community Effects

By the end of the 2.5 month experiment, several species of polychaete showed significantly lower densities and overall total number of taxa was significantly lower at both treatment plots compared to the control plot. However, total number of individuals, total oligochaetes and total densities of Scoloplos fragilis, Exogone hebes, Hydrobia totteni , showed no variation between plots.

Single pass of full sized scallop dredge (12 spring-loaded dredges, deployed either side in groups of 6 attached to two beams) along 300m transects. Video recordings before and after and survival studies of specimens in laboratory for 14 days.

Habitat Effects

Scallop dredging can alter the substrate composition. Stones and boulders (up to 60cm in length) overturned, small boulders piled against larger boulders, fragments of mudstone reef broken off, sand waves in the dredge path completely obliterated, suspension followed by settlement of fine sediments disturbed by the dredge and displacement of substrate (apart from mudstone, loose rocks brought to the surface and shovelled off the deck once the catch had been sorted). Overall there was a markedly changed appearance the most striking being the covering of all boulders and rocks with a fine coating of sediment. Chipping and movement of cobbles and boulders has implications for the habitat of juvenile crabs, particularly Cancer pagurus, which appears to inhabit the areas of soft mudstone. Of the habitats studied, area of sand waves was probably the least vulnerable to scallop dredging in the long term.

Species and Community Effects

Changes in species observed before and after dredging due to various factors; revealed by dredge as substrate overturned, dug out of substrate (eg Pomatocerus triquiter, Pecten maximus) or dislodged off the interstices eg Maja squinado; species hidden Porifera, destroyed Pentapora foliacea, injured or killed by action of dredge (adult crustaceans) and attracted by injured specimens in wake of the dredge Pollachus spp. crustaceans. Survival of dredged specimens in laboratory tanks showed surprising resilience of juvenile C. pagurus and Pholus dactylus which remained in the honeycomb mudstone, sea squirts died rapidly compared to controls and starfish exhibited comparable survival between experiment and control. No clear cut evidence in the case of P. foliacea and E. verrucosa but these most likely to suffer from being displaced as unlikely to re-establish themselves so mortality of these species seems likely.

Response of the whole system to dredging will depend on resettlement and growth of new stock and whether the substrate is suitable for this. The vulnerability of the system switching to another system would depend on importance of the species affected. If slower growing species with poor recruitment (eg E. verrucosa or slow growing but rapidly recruiting (eg P. foliacea) hold the system in its present form there is a high risk of complete change.

Clear differences in epifaunal communities between areas considered to be worked by mobile fishing gear and those not, however different sediment types in these areas is another influence. Reefs highly vulnerable to removal of epifauna and erosion caused by the action of the gear. Reefs with large boulders or severe topography which prohibits the use of fishing gear considered to be self protecting. Complex areas of sandy pockets, cobbles and boulders the size of which do not prohibit the use of rock hopper or spring loaded dredges, which support slow growing and numerous hydroids, anemones and corals, bryozoans, tunicates and echinoderms particularly vulnerable to highly mobile fishing gear. Recolonisation and recovery likely to be slow. .

Benthic impacts of oyster cultivation based on physico-chemical and biological parameters and the refinement for monitoring and assessment protocols. Assessment took place in an area that had been farmed for more than 8yrs and where mature oysters were present at the time of the survey. Seabed under the farm assessed in relation to 3 controls and spatial effects considered by sampling at three distances (farm parameter along two transects). In the farm sampling was carried out directly beneath the 1m wide racks. At each of the 11 sites three sediment cores were taken.

Habitat Effects

The rate of sedimentation beneath the cultivation racks was twice what was measured at adjacent sites. The silt/clay content of sediment trap samples beneath the racks was 96% compared with 83-89% at other sites. The organic content beneath the rack was higher (11.2%) than all the other sites outside of the farm perimeter which were not elevated (8.3-9.1%).

Species and Community Effects

For the control and outer transect sites the seabed consisted of a relatively low density assemblage, this included a variety of large-bodied individuals associated with undisturbed areas. However a progression towards the farm indicated that at 5m from the farm edge there was a moderate number of co-dominant species and underneath the racks themselves a high density of capitellid polychaetes.

An experimental study to quantify the effects of mussel Mytilus edulis culture on bird assemblages on an intertidal mudflat. Bird behaviour was monitored over two winters in an area of 4.31 ha, comprising of experimental mussel culture and control plots.

Habitat Effects

Species and Community Effects

Laying of the mussels had no effect on species presence/absence. Although no species were lost from the experimental plots, the bird assemblage in them changed.This reflected variation in the distribution of the 5 most abundant species. However, none of these key species declined in abundance following the laying of mussels. Curlew Numenius arquata and redshank Tringa totanus increased in abundance, although, oystercatchers Haematopus ostralegus did not.

Authors found that Oysters are direct trophic competitors of other filter feeders, and their presence modifies benthic–pelagic coupling by forcing a shift from pelagic consumers to benthic consumers. Increasing the surface area of cultivated oysters caused secondary production to increase, providing food for top predators (in particular juvenile nekton), reinforcing the nursery role of the mudflat in the ecosystem, and altering the species composition available to the top predators.

Location

Marennes-Oleron Basin, France Atlantic Coast

Fishing Types

Oyster culture systems.

Reviewed by

Sewell et al 2007

Full Reference

Beadman, H.A., 2003. Impact of mussel cultivation with special reference to the Menai Strait and Conwy Bay candidate Special Area of Conservation Countryside Council for Wales. CCW Contract Science Report No: 580.

Description

Review of the known and potential impacts of mussel cultivation, with particular reference to fisheries operating in two candidate SACs in Wales.

Habitat Effects

Mussel seed collection Seed mussels are dredged from the seabed using small, light-weight dredges. This takes place only when a sufficient amount of mussel mud has developed beneath the mussels to allow easy removal of mussel seed with minimal physical impact on the original substratum. As a result, it is thought that the original substratum is not significantly impacted by this practice. Any effect of seed collection is likely to be limited as the resource is naturally, regularly lost to disturbance events. Effects of faecal and pseudofaecal waste Bio deposition of these fine sediments leads to build-up of mussel mud to form mussel beds raised from the natural seabed. High organic content of deposits may lead to anoxic conditions and increased sulphate levels, but may represent an important food source for infauna. During harvesting, routine dredging and maintainance of mussel beds, plumes of sediment may be resuspended for up to an hour. This adds nutrients and oxygen consuming substances to the water column and may settle over the surrounding area, potentially having detrimental effects on any species sensitive to smothering. Changes in nutrient flux Mussels cycle nutrients through their own metabolism and through bacterial decomposition within the mussel bed. This leads to nutrient fluxes within the beds that tend to be higher than sediment without bivalve beds. The ecological consequences of these processes is not yet known.

Impacts of harvesting Because the mussel beds are not natural and are replenished soon after dredging with new seed mussels, direct impacts of dredging are minimal. Resuspended sediment plumes have the potential to effect species sensitive to high sediment loads and may release high levels of nutrients into the surrounding water. Hand gathering instead of mechanical harvesting will reduce this impact. However, access to the sites by foot my effect the site by trampling and increased disturbance to birds.

Species and Community Effects

Mussel seed collection. It is possible that the removal of mussel seed will remove a source of food for a number of predatory species, including several bird and fish species. In the Wadden Sea, removal of mussel seed beds has in the past had negative impacts on wild bird populations. Change in benthic communities Following seeding on a suitable substrate, mussels grow and form a secondary habitat composed of layers of mussels with accumulated mud and faeces. This can either enhance or degrade the infauna within or beneath the mussel matrix. The mussels can create a more complex habitat, and an organically enriched sediment capable of supporting a wide diversity of organisms. The mussels themselves may also provide a source of food for a number of predatory animals. Conversly, mussel beds can also reduce the diversity of infanal organisms through smothering, competition, anoxia and removal of larvae from the water by filter feeding. At high levels, these negative impacts of mussel beds can outweigh the positive effects and the impact that that these beds can have is dependant on the biomass of mussels in the bed. Studies in the Menai Strait indicate that impacts may impact surrounding sediments for hundreds of metres. Impacts of mussel beds on phytoplankton. Mussel beds can reduce phytoplankton biomass in the surrounding water column. Non-selective filter feeding can also have the effect of skewing natural communities towards smaller, faster growing species. Mussel beds can also increase phytoplankton growth through the recycling of nutrients into the water culumn. Increased food supply for predators It is possible that mussel beds have a positive effect on populations of predatory species, including oystercatchers, common starfish and the common shore crab by providing an extra food resource.

Study examining the effects of crab tiling on soft sediment, benthic communities. Three treatments were used and the experiment lasted for five months. 'Tile plots' had ridge tiles placed on them and were tended twice weekly to simulate bait collection. 'Trampling plots' Were walked over twice weekly, but had no tiles. And control plots were left untouched. Core samples were taken before treatments and at subsequent 7 week intervals. Tiles were 'fished' for crabs twice weekly and measurements and observations were made of all crabs found. Observations of other flora and fauna within each plot were also made.

Habitat Effects

Sediment composition was not effected by trampling nor the presence of tiles.

Species and Community Effects

Core sampling revealed a rapid decrease in number of individuals of all taxa under tiles during the first 45 days. This was attributed directly to the presence of the tiles as overall abundance increased in control and trampling sites during this period. The substantial decrease continued throughout the experiment. Although, abundance of individuals also decreased significantly in trampled sites, this was markedly less than reductions caused by the presence of tiles.

The number of taxa present was reduced under tiles, but not significantly in trampled plots. Neither tiles nor trampling effected species richness or diversity over the course of the experiment. Multivariate analysis of community showed that tiles had a greater impact on infaunal community structure than trampling. The presence of mature crabs declined from an average of five crabs per 10 tiles in the first month of the experiment to a much lower level at the end of the experiment. Crab tiles appeared to act as a refuge for juvenille (first year) shore crabs.

Peer review of a report (Beadman, 2003) examining the impacts of mussel seabed-lay bottom cultivation. The authors provide a review of the report, which includes identifying whether the impacts of this activity have been correctly identified and highlighting any potential impacts that may have been omitted from the 2003 report. Relevant points additional to those in Beadman (2003) are included below.

Habitat Effects

During a study in the Netherlands, the hypothesis that mussel seed removal increases the stability of sediments was tested. It was discovered that although increased stability was not observed, fished areas had the same numbers of mussels as unfished reference sites after a winter, indicating that the number of seed mussels taken by fisheries was roughly the same as was removed naturally by winter storms. However, it is also pointed out that studies elswhere indicate that if left, mussel seed is not 'lost' but dispersed to other locations and extraction from these beds may restrict the natural colonisation of other suitable sites. Removal of shell and seed mussels from areas of the seabed may also reduce natural recruitment of mussel in following years.

Cultivated mussel beds are removed regularly for harvesting and are therefore different to natural beds as far as long term changes to benthic communities are concerned. Cultivated mussel beds are thought to be effective in controling eutrophication by removing nutrients from the water.

The effect that the resuspension of sediment caused by dredging is likely to have on the surrounding benthic community will certainly be influenced by the tidal stage at which dredging takes place. Shellfish re-layed from some locations may act as vectors for the introduction of harmful non-native species.

Density of common macrofauna decreased at dredged sites and some significant differences still apparent after 3 months. At both sites more than 50 percent of the common taxa showed significant effects. Differences in recovery process likely to relate to differences in initial community composition and to differences in environmental characteristics. Authors consider the effects recorded were conservative as commercial fishermen work over much larger areas and repeatedly dredge the same area in any one fishing trip.

Study examining the effects of commercial baitworm harvesting (digging) on the semipalmated sandpiper. Studies were made of feeding efficiency in dug areas, compared to undisturbed areas and these were correlated with food availability.

Habitat Effects

Species and Community Effects

Foraging efficiency decreased by 68.5% in dug sediment, corresponding to observed reductions in prey density. All the significant, negative effects of baitworm harvesting on Semipalmated Sandpiper foraging behavior and on the density and age structure of their principal prey, C. volutator, were realized after only one season of digging.

Management was based on the Pearl River stage, rainfall amounts and faecal coliform counts, where the information had to be tracked down via official reports, internet and direct observations. If levels increased too much then areas of the fishery would close. The study aimed to design a GIS-based decision support tool that will ease the management of shellfish fisheries within the state.

Report develops a predictive model to explore the effect of different shellfishery management options on the mortality rates of the migratory shorebirds that feed on shellfish on intertidal wintering grounds in Europe. Effects incorporated include disturbance and reduction of abundance of the shellfish stocks. Application to the Exe estuary was successful in predicting levels of oystercatcher winter mortality in previous years

Habitat Effects

Species and Community Effects

Main conclusions were:

Given a number of conditions it is possible to exploit shellfish stocks without increasing the winter mortality of shorebirds.

Effects of a given intensity of shellfishing depends crucially on local conditions of the climate and the general abundance of food.

Methods of shellfishing which disturb birds can be significantly more damaging to the bird?s chances of survival.

Numbers of birds using alternative food sources is an early warning that a change in shellfishery practice is beginning to have an effect on the birds.

Key factor in determining the impact is the proportion of the shellfish stock that is affected.

Cumulative effects of small increases in shorebird morality in winter can over a period of years greatly affect stable population size.

As fishing effort increases, shorebird mortality may be hardly affected initially but then may suddenly increase dramatically once a threshold level of fishing effort has been reached.

Location

Model tested on Exe estuary

Fishing Types

Various (see further notes). Disturbance (general). Competition for resources (general).

All the epifaunal invertebrates counted were less abundant in the heavily trawled area. No differences were found in the number of infaunal crustacean species but there were more polychaete species in the lightly trawled area every year, implying that high levels of trawling can reduce biodiversity. This also suggests that high-intensity trawling favours opportunistic species.

High numbers of ophiuroids and the amphinomid polychaete Chloeia pinnata in the highly trawled area may be because they can pass through net mesh unscathed and then benefit from feeding on those organisms that the net crushes or kills. C.pinnata was also found to be the most common invertebrate in the diet of several commercially important flatfish species in both areas suggesting that certain prey species and commercially important fish may be enhanced by some level of trawling disturbance.

Description of cockle fishing practices in Morecombe Bay. The author describes his observations of changes to the cockle industry, also management response and social issues related to the changing cockle fishery. The report describes how the cockle fishing industry changed from a relatively small scale fishery prior to 2002 to a large scale fishery with over 400 hand gatherers working beds at the peak of activity. Large vessels, usually used for suction dredging were dried out on cockle beds and used to collect and transport large tonne bags of cockles gathered by large numbers of people from the shore. At times, up to four vessels were operating at a time.

Habitat Effects

Species and Community Effects

Although no environmental effects are described in this article, the author notes that the area is designated as an SPA for its important bird life and an SAC for other wildlife and effects that the fishing activity has on these features have implications for fisheries management.

Based on summary in Fowler (1999) (in main references). Study examining the effect of overturning boulders for the collection of peeler crabs etc.

Habitat Effects

During periods of reasonably low tides at both study sites, 3,000 rocks were overturned. An unknown number of these involved repeated turning of the same rocks. No 'serious' collector was seen to replace rocks in their original position. The chief result of this activity was thought to be the loss of habitat stability.

Species and Community Effects

The loss of habitat stability due to the turning of boulders was thought to affect the range of species present.

Based on summary in Fowler (1999) (in main references). Study examined the impact of boulder turning when searcing for pealer crabs.

Habitat Effects

Up to 90 percent of all boulders along a transect at the study site could be turned over witin a two week period and during the summer, some boulders may be turned 40-60 times. 60 percent of boulders are not replaced in their original position. Larger boulders, 'upended' and not over turned completely are most likely to be left as they were found.

Report addresses the goals of the fishery considering sustainability, the environment, use and management. Strategies were then put forward to meet the various objectives and at the end of 2007 these strategies will be reviewed to see how effective they were at meeting the objectives.

Brief summarises the current situation of the fishery under three headings: i) biological information, ii) social, cultural and economic information and iii) management and service. The brief also supports the development of a management plan for the Foveaux Strait dredge oyster fishery with various management measures.

Harbour porpoises are taken throughout their range and several populations are in decline, at least partly as a result of gill net entanglement. In the eastern North Atlantic substantial numbers are caught in gill nets in most areas. Highest known takes in Norway, Sweden and Denmark. UK also has substantial takes in gill nets as well as other fisheries.

There are reports of harbour porpoise being caught in cod, salmon and whitefish gill nets off the Scottish coast, and in salmon drift nets and inshore set nets off NE England.

Gill nets (which include set nets, drift nets and trammel nets) are considered to represent the single most important threat to porpoises as a group. Most porpoises have substantial problems with them. Harbour porpoise, for example, are found primarily in shallow waters, mostly nearshore which is the area where this form of fishing is generally practised.

A stratified sampling study was developed to describe the spatial variability of the diversity and structure of macro- and mega-faunal communities in the Bay of Biscay, to identify any changes that appear to have occurred as a result of fishing pressure since the sixties.

Habitat Effects

Species and Community Effects

Three distinct communities were identified. In each community, the epifauna showed less diversity, species abundance and biomass in the areas exposed to higher levels of fishing effort, while endofauna seemed not to be affected.

Experimental dredging studies on hard sand and a soft mud compared to an area of no dredging.

Habitat Effects

Species and Community Effects

Experimental dredging studies on hard sand and a soft mud compared to an area of no dredging showed a significantly reduced level of eelgrass biomass and shoot number on both hard and soft seabed. The seagrass was more susceptible to damage (all shoots removed) in the latter case whereas on hard seabed about 15 percent of the eelgrass per core remained.

Study of bottle-nosed dolphin interactions with gillnets set for Spanish mackerel Scomberomorus maculatus. Observations of dolphin behaviour around the net were recorded using a digital video camera suspended directly above the net from a helium balloon. 30 replicate sets were used and observations were later made in a laboratory.

Habitat Effects

Species and Community Effects

Encounters were observed between dolphins and 24 of the 30 nets. Direct interactions were recorded with 19 of the 30 nets. Multiple encounters and interactions were recorded at several of the nets. Dolphins most commonly avoided the nets, but there were several incidences of dolphins 'patrolling' the edge of the nets and occasionally taking fish (depredation). Despite this level of interaction, there were no recorded incidences of dolphin entanglement

Study examining the response of bottle-nosed dolphins to bottom set gill nets with attached, functioning and non-functioning acoustic pingers. Dolphin movements were monitored around commercial gill nets every morning of the study period. Control nets had attached acoustic pingers with no power.

Habitat Effects

Species and Community Effects

59 groups of dolphins were observed during the study. No dolphins were caught in the nets during the study. The number of dolphins observed per hour and the closest observed approach to the net did not differ between treatments. the number of dolphins entering a 100 m radius of the net varied significantly between treatments and was lower with the active pinger. Most dolphins appeared to be aware of the net irrespective of treatment and some dolphins fed on fish trapped in the net.

Study of marine mammal bycatch associated with 11 pelagic trawl fisheries operating in the North East Atlantic. Observers were placed on board vessels to monitor marine mammal bycatch, observations of marine mammals and number of trawls. Some post mortem analysis was also carried out.

Habitat Effects

Species and Community Effects

Four grey seals were landed in the Irish herring fishery operating in the Celtic Sea. Common dolphins, white sided dolphins and possibly one bottle-nosed dolphin (18 total) were caught in the Dutch horse mackerel fishery, French Hake and Tuna fisheries and French Bass fishery. The catch rate of seals in the Irish herring fishery was 0.0513 seals per tow or 0.0396 per hour of tow. The mean +_SD dolphin catch rate for all fisheries combined was 0.048+_0.013 per tow (one dolphin per 20.7 tows), or 0.0185+_0.0019 per hour of towing (one dolphin per 98 h of towing). Cetacean bycatch was highest in the French sea bass fishery and lowest in the French tuna fishery.

all towed gears which exploit bottom-living species disturb the sediment and may therefore have an impact on the structure and processes at the seabed. Grain size distribution, sediment porosity and chemical exchange process are properties which may be affected. Another direct consequence is displacement of boulders which would otherwise be a surface for epifauna. A direct consequence of disturbance is an increase in suspended sediment load and the possibility of net transport of finer sediments. Resuspension may also influence uptake or release of contaminants, a shift in sediment-water exchange eg of nutrients. Reworking of sediments may result in burial of organic matter. Gears which disrupt the sediment most are beam trawls and shellfish dredges but method of rigging can have a profound effect on the level of disturbance.

Species and Community Effects

Box cores revealed extensive changes to infauna before and after trawling. Significant reduction in burrowing sea urchin and the density of tube-building polychaetes. Survival rates for infauna and epifauna caught in net of beam trawl were high for starfish, many molluscs and crabs but poor for Arctica islandica. Trawl-caught whelks and hermit crabs largely unaffected. These results suggested that a relatively high proportion of some benthic species can be killed in the path of a beam trawling. In relation to scallop dredging epibenthic mortalities can be marked. Effects on seabed and benthos depend on substrate type, hydrographic features and community structure as well as the design and operation characteristics of the gears. Seabirds have been killed in gill and other static nets, no comprehensive studies of entanglement in the North Sea but available evidence indicates that it is likely to occur for diving birds in areas with fixed net fisheries. Gill net fisheries in some places have had a high by-catch of diving birds. Seals may be caught in gill nets, fyke nets and fixed nets for salmon. Gill nets killed the most cetaceans, catch rates varying seasonally. Around the British Isles several species of small cetacean have been reported as incidental catches but in the North Sea reported by-catches of species other than harbour porpoise are rare. As well as catch, fishing operations cause incidental mortality of fish which escape from the gear.

Gill nets, tangle nets and traps may continue to fish for some time after being lost of discarded. Length of time depends on factors such as current speed and fouling. On the bottom multifilament nets remain tangled, monofilament nets may, once clear of fish remains and crabs, disentangle, return to an upright position and resume fishing. Over time they build up an encrusting layer of marine organisms and become more visible to fish. Fragments of nets of all types may also entrap seabirds and marine mammals. Direct effects of fishing compared with the effects of other anthropogenic influences and natural processes also discussed, along with long-term effects of fishing activities. In the long term there may be changes in the feeding relationships of organisms, changes in the genetic makeup of populations and other changes such as in the habitat. The mix of direct and indirect effects makes it extremely difficult to establish causal relationships between the amount of fishing and observed long-term population changes. Long-term cascading changes in community structure may occur if 'keystone' populations are adversely affected by fishing, leading to marked changes in the pattern of predation and or competition. One general effect that has been suggested for benthic communities is that overall productivity may increase due to long-lived slow growing taxa being replaced by smaller faster growing taxa whose populations are better able to respond numerically to continued disturbance. Such shifts, it has been suggested, could lead to changes in other community parameters such as species diversity. However, not all levels of disturbance will necessarily result in lower community diversity. Current ecological theory supports the idea that intermediate levels of disturbance would result in an increase in diversity.

Video and photographic survey of sites with varying degrees of fishing disturbance along transects during two experimental cruises to the area.

Habitat Effects

Emergent colonial epifauna provided a complex habitat for a number of invertebrates and small fish at undisturbed sites. Bottom fishing was found to remove this epifauna, thus reducing the structural complexity and species diversity of the benthic community.

Species and Community Effects

For photographed sites, significant differences between disturbed and undisturbed areas were found for; the percentage of the bottom covered by "bushy, plant-like organisms" and colonial worm tubes and the presence or absence of encrusting bryozoa. Colonial epifauna were conspicuously less abundant at disturbed sites.

The paper examines spatial differences in the distribution of bycatch assemblages from scallop fishing grounds. High-resolution fishing effort data was extracted from fishermen's logbooks and used to identify areas with varying levels of disturbance. Species composition of experimental trawls at different sites over time was analysed and compared.

Habitat Effects

Species and Community Effects

Species diversity and richness, total number of species and number of individuals all decreased significantly with increased fishing effort, as did total abundance, biomass and production of most major individual taxa investigated. Species dominance increased with fishing effort. Bycatch assemblage structure was more closely related to fishing effort than any other environmental variable examined.

Location

North Irish Sea, around the Isle of Man.

Fishing Types

Scallop dredge.

Reviewed by

Sewell & Hiscock 2005

Full Reference

ICES, 2006. Report of the Working Group of Ecosystem Effects of Fishing Activities (WGECO), 5-12 April 2006, . 174

Description

The working group reviewed the impact that fishing gear had on all components of the ecosystem and described the distribution of the following fishing efforts: beam trawls, otter trawls and small-meshed fisheries.

Habitat Effects

Dredging, otter trawling and beam trawling all involve contact with the seafloor and therefore causing the removal of large physical features as well as altering both structural biota and habitat complexity. The impact of otter trawling is considered to be less than that of beam trawling, with dredging being the most disruptive to both benthic habitats and processes.

Species and Community Effects

Dredging: no evidence of concern was presented over the by-catch of commercial and non-target fish species and no impact on marine mammals or seabirds was presented.
Otter trawling: there have been a few records of seabirds and marine mammals becoming caught in otter trawls as by-catch, these species have been recorded feeding in the nets and on fish that escape through the mesh.
Beam trawling: no evidence of concern was presented over the by-catch of marine mammals or seabirds.

The study involved the review of both published and unpublished data to compile a long-term data set to assess the changes within the benthic communities of the North Sea at five fishing grounds over the past 60 years. The five fishing grounds were: i) Dogger Bank, ii) Inner Shoal, iii) Dowsing Shoal, iv) Great Silver Pit & v) Fisher Bank.

Habitat Effects

Species and Community Effects

Results indicated differences between the 1920s and post 1985 samples at three of the five sites; this shift in benthos has coincided with an increase in the fishing power of the fleet. Although there were clear differences over time at three of the sites the changes did not appear to be as a result of a decrease or disappearance of sensitive species or an increase in opportunistic species. Instead the changes appeared to be as a result of changes in abundance of many taxa as opposed to a large-scale loss of sensitive organisms. Despite the fact the study did not indicate a loss of sensitive species it does however indicate that increases in fishing effort can alter the benthic community of the seafloor and therefore must be considered by fisheries managers.

Arctica islandica was used as an indicator species for investigation of long-term effects of beam trawling intensity in the North Sea with sampling clusters in the NW, mid-west and SE.

Habitat Effects

Species and Community Effects

A high incidence of damage found on shells of Artica islandica from highly fished areas particularly in the south eastern North Sea. In specimens with two values only 10 percent of the SE North Sea specimens were undamaged and in other areas around 40 percent were undamaged. 80-90 percent of the damage found on posterior ventral side of the shell was explained by the orientation of the living shell in the upper sediment layer and the horizontal motion of tickler chains. Observed trends in the occurrence of shell scars per year show a striking coincidence with the increased capacity of the Dutch beam trawling fleet since 1972. Another effect may be on age frequency distribution as juveniles (1-4cms) were rarely found in the SE North Sea. Less resistance to damage may be a factor although the authors indicate that other researchers have contradictory information on this.

Towed fishing gears such as bottom and beam trawls physically disturb the seabed causing alterations in microbial communities, resuspension of particles, nutrients and pollutants and the relocation of stones and boulders. Inshore fisheries have led to destruction of reefs built by species such as the polychaete worm Sabellaria or by calcareous algae. Fishing has led to structural changes in habitat that have resulted in changes in species assemblages

Species and Community Effects

Fixed nets such as gill nets are more likely to entangle non-target species. Diving seabirds are especially vulnerable to entanglement in fixed nets such as gill nets. No evidence that mortality due to entanglement has precluded the observed increase in population size of many species of seabirds which has taken place during this century in the North Sea. Harbour porpoises especially vulnerable to entanglement in gill nets. Recent estimate of the by-catch of the Danish gill net fishery in the eastern North Sea gave an annual by-catch of 4629 porpoises. Incidental by-catch could be a significant contributing factor to the overall decline harbour porpoise abundance in European waters. Seal populations have been able to sustain or increase their populations whilst subject to fishery induced mortality. No species exists in isolation, fishery-induced changes in the density of one species will have repercussions on its predators, prey and competitors.

Heavy towed gears in contact with the seabed can kill or injure animals living in the top most layers of sediment. The percentage of benthic organisms caught in a beam trawl which die varies from zero for hermit crab, whelks and starfish to 100 percent for shells such as Artica islandica. Beam trawl is the most important fishing gear which penetrates the seabed. General fisheries generated mortality results in reduced abundance of long-lived benthic species and increased abundance of short-lived species. By-catch and offal produced by gutting the fish at sea thrown overboard provides food for seabirds and other scavenging animals. Changes in the amount of discards may affect the relative and absolute abundance of various species of seabirds. Increased abundance of scavenging seabirds since the start of the century. Large or unattractive discard items will fall to the seabed where they can become available to sub-surface scavengers.

Fishing produces litter in the form of lost gear and other waste comparable with that produced by shipping in general. Litter from fishing such as lost or discarded nets may entrap seabirds and mammals.

Changes in the balance of the benthos, particularly the loss of Sabellaria reefs and oyster beds attributed to over-fishing and trawl damage. Comparable shifts in dominance with certain polychaete species commonly favoured over more vulnerable groups such as echinoderms anticipated at regularly fished sites, and is, in principal, reversible. Recent trend towards the deployment of larger, heavier demersal fishing gear enhances the possibility of benthic changes in intensively fished areas. Shrimp fishery in Wadden Sea observed a long term decline in the number of by-catch species notably Carcinus and Pomatoschistus spp. Biomass of by-catch remained constant with compensating increase in dab, sprat and cod.

Location

North Sea

Fishing Types

Trawling.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Philippart, C.J.M., 1996. Long-term impact of bottom fisheries on several bycatch species of demersal fish and benthic invertebrates in the southeastern North Sea ICES . C.M. 1996/Mini 6

Description

Long term historical record (1945-1981) of by-catch from an area of the North Sea to the Northwest of the Netherlands at Zoological Station in Den Helder.

Habitat Effects

Species and Community Effects

Bottom fisheries have a considerable effect on many by-catch species including demersal fish and invertebrates. Numbers of by-caught fish and invertebrates related to changes in fish gear and effort of bottom trawlers. Catchability of beam trawlers 10x higher than otter trawls. Model of bottom fisheries shows that bottom trawling has reduced the abundance of several demersal fish and invertebrates to very low levels within 35 years.

Data from a study of scavenging seabirds in the North Sea and review of literature on quantities of discards. Fishery waste from North Sea fishery is important to seabirds. The sources evaluated here are demersal trawlers and seiners catching gadoids, pelagic trawlers and seiners, and beam trawlers. Authors estimate quantity available amounts to around 62,800t offal, 262,200t roundfish, 299,300t flatfish, 15,000t elasmobranchs and 149,700t benthic invertebrates per year. Beam trawls have the highest rates of discards of fishing fleets in the area. Discard fraction is dominated by flatfish which are less favoured by seabirds potentially supported by fishery waste in the North Sea estimated to be roughly 5.9 million individuals in an average scavenger community.

Habitat Effects

Species and Community Effects

Discards and offal may easily support all scavenging seabirds in southern and southeastern sub-regions of the North Sea for example but only half in the northwest region.

Location

North Sea

Fishing Types

Various (see further notes). Competition for resources (general).

Reviewed by

Gubbay and Knapman, 1999

Full Reference

de Groot, S.J., 1984. The Impact of bottom trawling on benthic fauna of the North Sea. Ocean Management, 9, 177-190

Description

Review of impacts of bottom trawling

Habitat Effects

Effect of trawls will be influenced by substrate. Visibility of markings depend on substrate and currents and depth of penetration up to 30mm on muddy ground and 10mm on sandy ground.

Author speculates that it is not unlikely that in the long-term a shift in species and numbers may occur as has been found in the German Wadden Sea where polychaetes are on the increase and molluscs and crustaceans in decline but that this is unlikely to have a negative effect on fish stocks. Large quantities of benthic animals become available as food source for fishes. Temporary covering due to sand movement is not exceptional and they will survive, and a shift in species distribution from one group or groups of animals to another cannot be ruled out in the long-term. Author comments that as this shift is, in principle, reversible it constitutes no major threat to benthic life.

Location

North Sea

Fishing Types

Beam trawl.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

McGlade, J.M. & Metuzals, K.I., (ed. M.J. Kaiser & S.J. de Groot) 2000. Options for the reduction of by-catches of harbour porpoises (<em>Phocoena phocoena</em>) in the North Sea. , In: The Effects of Fishing on Non-target Species and Habitats: Biological, Conservation and socio-economic issues, pp. 105-117.

Description

The paper examines management options to reduce harbour porpoise by-catch from North Sea fisheries, particularly bottom-set gill-net fisheries. The paper included a study of the by-catch associated with UK and Danish, bottom-set gill-net fisheries. The UK section of the study was based on 27 Grimsby gill netters, targeting cod. Interviews were undertaken, along with observations made from on board fishing vessels. The Danish fleet analysis was based on interviews with 30 gill-netters targeting a mixture of species, from various fishing ports in Denmark.

Habitat Effects

Species and Community Effects

Estimates of harbour porpoise bycatch from the Grimsby gill-net fishery ranged from 81 to 193 per year over the period between 1990 and 1997, based on observer data and detailed spatio-temporal analysis of the fishery. Catches ranged from 95 to 202 for 1997 to 1998 based on interviews with skippers. Bycatch from the Danish Gill-net fisheries was estimated at between 3500 and 4500 per year in 1998.

The paper examines the provision of discards and offal as a food source for sea birds, Overfishing of large predatory fish and overfishing of small fish by commercial fisheries. The aim was to explore the hypothesis that the recent increased range of many seabirds in the North Sea was influenced by commercial fisheries.

Habitat Effects

Species and Community Effects

Larus gulls used discards to a considerable extent. Black-legged kittiwakes largely ignored discards and prefered to feed on small, live fish. Non-breeding birds used discards most frequently. Nesting birds made a greater effort to feed on natural resources this may be related to reduced breeding success resulting from a diet consisting of high amounts of discards. The authors found no evidence that seabirds profited from the removal of predatory fish. Several examples show how overfishing of certain stocks can reduce the reproductive output of some seabirds.

The aim was to show if there was a link between discard availability and discard use by a generalist predator and scavenger the great skua and test the use of other prey species in its diet. Proportions of discards of white fish species within the diets of great skua were compared with data from ICES on the estimates of quantities of fish discards.

Habitat Effects

Species and Community Effects

There was a positive correlation between discard estimates and the importance of both whiting and haddock in the great skuas diet. Results indicated that declines in discard availability have coincided with declines in sandeel biomass, which has lead to the prey switching tendency of the great skuas. Although the great skua may not suffer population declines as a result of declines in sandeel numbers other seabird may, particularly as the great skua can switch from discards to sandeels to seabirds. Models indicated that a 5% increase in birds in the great skuas diet is equivalent to an additional 1,000 northern fulmar or 2,000 black-legged kittiwake.

Location

North Sea

Fishing Types

Competition for resources (general).

Reviewed by

Sewell et al 2007

Full Reference

Furness, R.W., 2002. Management implications of interactions between fisheries and sandeel-dependant seabirds and seals in the North Sea. ICES Journal of Marine Science, 59, 261-269

Description

Study to review the interactions between industrial fisheries (sandeel fishery) and populations of top predators in the North Sea (seabirds and seals), where the fishery has continued to grow alongside growing populations of sandeel-dependant seabirds and seals.

Habitat Effects

Species and Community Effects

Increases in the sandeel fishery have been recorded since the fishery began in the 1950s, increasing rapidly during the 1970s to 800,000t by 1977, since then landings have rarely fallen below 600,000t. This suggests that bird or seal populations may show changes during the rapid increase in landings. However, seal population numbers were increasing before the fishery began and have continued to increase alongside the fishery. From 1969-1987 four species of breeding seabirds in the North Sea showed declines (herring gull, common tern, roseate tern and great black-backed gull), however increases were seen in 15 other species by 10% or more. Seven species that increased by 100% or more fed extensively on sandeels while breeding.

Two studies, examining both the long-, medium- and short -term effects of a Nephrops otter trawl in the Irish sea. To calculate short term effects, two sites were used, a heavily fished offshore site and a less frequently trawled inshore site. Day grab sampling was used to identify initial species assemblage. Each track was trawled twice and a further set of day grab samples were taken within 24 hours to establish direct impacts of the trawl. Long-term effects of fishing were investigated by comparing two unfished shipwrecks, one nearshore and one offshore with nearby fishing areas.

Habitat Effects

Species and Community Effects

Offshore: There was a clear differend between unfished areas and fished areas. The fished site had lower species diversity, number of individuals and biomass compared to the area around the wreck (unfished area). In addition, large specimens of some molluscs and echinoderms were found at the unfished site, but not at fished sites.

Inshore: Most species showed a statistically insignificant decrease in numbers immediately after a trawl. Although most species of polychaete increased in numbers (generally small oppertunistic species or large scavangers). Number of individuals and biomass decreased significantly between the wreck site and the fishing ground prior to experimental trawling. In fished areas, number of species, biomass, species-richness and shannon's diversity index decreases significantly following experimental trawling. 49 species were found at the unfished site that were not found at fished sites, whilst 19 were found only at fished sites.

A study comparing the impact of a benthic otter trawl with a semi-pelagic otter trawl, fished approximately 15 cms above the seabed. Repeated trawls were undertaken in marked areas and the effects on macrobenthos (mainly sponges, soft corals and gorgonians) were recorded, by measuring by-catch and using video survey.

Habitat Effects

Species and Community Effects

No measurable effects were recorded following semi-pelagic trawls, whereas demersal trawls resulted in reductions to the density of benthic organisms growing higher than 20 cm from the seabed of 15.5 percent on each tow through the site.

Paper reviews different types of demersal gear and the effect these gears have on the marine ecosystem in the waters of north-western Europe.

Habitat Effects

Species and Community Effects

The extent of by-catch can indicate how much benthic communities are being disturbed. The occurrence of the heart urchin Echinocardium cordatum and the bivalve Arctica islandica indicates that the trawl has penetrated to a depth of at least 6cm into the hard sandy substrate. Fishing activities in areas of fragile or long-lived epibenthic invertebrates will have implications for the diversity of the community, the loss of biogenic reefs and their replacement by small polychaete communities can have implications. The reef building species, for example Sabellaria spinulosa and maerl, form microhabitats for other species, so the destruction of the reefs can result in the loss associated species. Scavenging species benefit from trawling activity and are often associated with trawled areas feeding on the remains of both damaged and discarded fauna.
In communities that are adapted to physical disturbance the effects of fishing disturbance are likely to be short-lived when compared to communities that are rarely disturbed by natural processes.

ICES 1996. Report of the Working Group on Ecosystem effects of fishing activities ICES . C.M. 1996/Assess/ Env:1. Ref: G.

Description

Update on studies relating to areas closed to fishing.

Habitat Effects

Species and Community Effects

Comparison of community structure in areas of high and low scallop dredging on northern Georges Bank shows undredged sites had higher densities of shallow burrowing and epibenthic species, more abundant Modiolus modiolus and more abundant small fish. Hard-shelled molluscs were equally abundant at dredged and undredged sites as well as scavenger species suggesting that scavenger abundance was not food limited. No consistent differences in mean size and weight of species between dredged and undredged sites. Many polychaete species were only abundant at the undredged sites because of the complex habitat there. Habitat complexity was higher at the undredged sites due to present of Filograna implexa, bushy bryozoans and hydroids.

Closed area (from 1989) of scallop ground off Port Erin, Isle of Man is being used to assess environmental impact of scallop dredging. Benthic community and physical habitat has been compared with adjacent areas since 1994 and two plots within the closed area experimentally dredged at 2 month intervals. Results to date show differences in the epifaunal communities including greater species consistently more abundant in undredged areas. Further analysis shows this was due to absence of dredging and not variations in sediment or depth. Overall higher densities of shallow burrowing and epibenthic species at the undredged sites but particular species noted for their vulnerability to dredging eg A. digitatum, Anseropoda placentaLuidia sarsi, Cellaria fistulosa and E. esculentus. There was no evidence of longer-lived benthic species at undredged sites but this was not surprising due to relatively short time since effective closure of the area. Scavenger species were common at both dredged and undredged sites with A. rubens consistently more abundant on the dredged sites. Ratio of polychaetes to molluscs was lower at the dredged sites and may be due to greater habitat complexity in the closed area although authors also note that infaunal bivalves were probably not adequately sampled.

Location

Northern Georges Bank, NW Atlantic
Port Erin, Isle of Man

Fishing Types

Scallop dredge.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Description

The review aimed to identify, evaluate and then recommend various options for the development and management of the inshore fishery within Northern Ireland.

Philippart, C. 1996. Long-term impact of bottom fisheries on several bycatch species of demersal fish and benthic invertebrates in the southeastern North Sea. ICES.

Description

Analysis of bycatch of 7 fish and 10 invertebrate species taken in otter and beam trawls in an area north west of the Netherlands which were registered annually between 1945 and 1983. A fisheries catchability model is developed using this data.

Habitat Effects

Species and Community Effects

For species with reliable field data the model results on long-term trends in abundance were in agreement with observations eg. considerable decrease in abundance of Roker and Common skate off Dutch coast between 1951 and 1960. Model also suggests that decline of landings of greater weever in early 1960s often considered to be due to severe winter and/or introduction of beam trawlers should also be attributed to effects of otter trawling. Most differences could be related to changes in gear and fishing effort with otter trawlers catching relatively more fish than invertebrates and beam trawlers catchability ten times higher than that of otters for all species considered.

Model estimates suggest that bottom fisheries had a considerable impact on the abundance of several bycatch species even before the Second World War.

The report documents the distribution of deep water coral reefs off the coast of Norway and the ways that they are effected by fishing activity. The authors reviewed available published and non-published information, obtained information from fishermen and carried out surveys using ROV equipment.

Habitat Effects

The authors documented extensive physical damage to reefs in all but one survey site caused by trawling activity. Trawl impacts included complete destruction of reef structures, removal or displacement of reefs by trawlers and scouring of reefs and the surrounding seabed by otter boards and the trawl net. Impacts of passive fisheries were also observed. Ghost fishing gill nets, anchors and other fishery related debris were found on several reef areas. The authors estimate that 30 - 50 percent of coral reef in this area has been damaged by fishing activity.

Gurnards and whiting aggregate over beam tracks to feed on animals damaged by the beam trawl or on other scavengers that are attracted to the trawled area. There was a particularly clear increase in the proportion of the amphipod Ampelisca spinipes in their diets and some mobile invertebrate scavengers such as Pandalus spp. only occurred in diets after the area was fished. Number of prey items eaten by gurnards and whiting increased after trawling. Dogfish did not increase their intake after trawling but did take Pandalus spp. and Crangon spp. only after the area had been trawled.

Results suggest that fish rapidly migrate into the area to feed. Additional resources such as those made available by trawling, may favour certain species that exhibit opportunistic feeding patterns such as gurnards and whiting.

Four experiments were carried out in the north Atlantic to assess the effectiveness of three mitigation measures during longline setting in a commercial longlining fishery. The measures assessed in reducing seabird bycatch were: bird-scaring line, underwater setting and a shooter. During each haul the number of marketable species and seabirds taken by each fleet were counted.

Habitat Effects

Species and Community Effects

Bycatch of seabirds was reduced by all three methods, although the difference for the line shooter was not significantly different. The clearest difference was using the bird-scaring line. During the course of the experiments 185,000 hooks were set using the bird-scaring line and only 2 birds were caught, whereas when the control line were set with a similar number of hooks 205 birds were caught. The bird-scaring line was also the most effective a reducing bait loss when the lines were set when compared with both the control and other two methods. Catch rates of the target species was also higher when using one of the mitigation measures than using no measures.

Comparative study of dredged and undredged sites to investigate effects of suction dredging on razor clam.

Habitat Effects

Species and Community Effects

Undredged site was characterised by an absence of small razor clams, contained the largest individuals, and a higher density of razor clams. At the dredged site the population had changed considerable in the 7 years of spasmodic dredging. The most notable differences were the absence of a middle size range of clams and a decline in the number of large razor clams. Shells from the dredged site hand considerably more disturbance marks/damage to the outer shell layer than at the control site with 70 percent showing the highest level ie. Deep clefts in the outer shell layer embedded with sand grains.

Observations of the reburying of razor clams collected by airlift and subsequently released onto the surface of the sediment suggested that they are highly vulnerable to attack from predatory crabs and will experience a high level of mortality after removal.

Study examining the effects of a trawling disturbance on a soft sediment ecosystem. The study area had not been trawled for 20 years prior to the start of the experiment. Both macrofauna and biogeochemical data was collected quarterly for 1.5 years before experimental dredging. Post trawl samples were taken for a period of 6 months.

Habitat Effects

Sediment porosity reduced significantly immediately after the trawl, but had returned to levels similar to control plots within four months. Chlorophyl a content of surface sediments was elevated significantly following the trawl.

Species and Community Effects

Immediately after trawling, species abundance, number of species and species diversity decreased significantly in the trawled area. Several species of bivalve and polychaete were found to be particularly sensitive, whilst a species of carnivorous nemertea was found to be resistant, probably on account of its ability to actively seek out freshly dead and dying organisms.

Simulated hand raking for cockles at 18 plots (six control, six small and six large plots), using rakes with 10cm long teeth. Sediment was disturbed in a manner similar to that of commercially deployed hand rakes. Infauna and sediment characteristics were sampled 1, 14, 56 and 503 days after raking once at low tide. The authors examined the effects of the raking disturbance on non-target species, associated with the cockle beds and direct impacts on under-sized cockles.

Habitat Effects

Species and Community Effects

Hand raking led to a three-fold increase in the damage rate of under-sized cockles, compared to control plots. There were community differences between both study plots and the control plots after 14 days. Small plots had recovered after 56 days, but large plots remained in an altered state. The authors concluded that the effects of raking were unlikely to persist beyond a year unless long-lived species where present.

Large scale investigations on soft sediment communities depth between 12-15m, 2km offshore. Six vessels towing 3m wide commercial ?Peninsula? dredge with scraper/cutter bars not extending below the dredge skids. Site dredged for 3hrs day-1 over 3 days covering the dredge area at least twice. Dredging intensity was typical of local commercial fishing intensity.

Habitat Effects

Typically top 2cm of surface sediment disturbed but up to 6cm. Observations 8 days after dredging revealed seabed formations such as pits and depressions filled in and mounds formed by burrowing shrimps removed. Parallel tracks from dredge skids apparent after dredging. Physical changes in the seabed still apparent one month post-dredging. Six months post dredging most physical features reformed (abundance and size of callianassid mounds similar to those present before dredging) however some flattened areas still apparent. No physical differences between dredged and control sites after 11 months.

Species and Community Effects

Ther number of species in dredged areas decreased significantly. Maximum impact did not occur immediately after dredging suggesting some indirect ecological changes such as uncovered organisms becoming more vulnerable to predation by invertebrates and demersal fish. Most species decreased in abundance by approximately 20-30 percent in the 3.5 months after dredging. The duration of the decrease in abundance species varied, with effects still apparent in some species after 8 months and in two species up to 14 months although this was possibly due to undersampling in the pre-impact period. 11 animals were not found in the sample area after dredging, mostly sedentary and therefore unable to re-establish except by larval recruitment.

Susceptibility to dredging not correlated to feeding type or rarity. Fragile groups such as nemerteans were greatly damaged by dredging, polychaetes probably cut and killed by passing dredge. Other species may have been affected by high rates of dredging induced sedimentation, which may be 2-3 orders of magnitude greater than storm produced sedimentation, or buried when depressions filled in. Two species showed significant increase in abundance following dredging (Diamorphostylis cottoni and Oedicerotid sp.) whereas the isopod Natalolona carppulenta decreased sharply and then increased to be consistently higher on the dredged plot for 8 months possibly due to greater availability of prey.

Seasonal and interannual changes in community structure much greater than those caused by dredging. Long-lived and slow recruiting epifaunal species (eg sponges and ascidians) likely to be particularly vulnerable to dredging. Long-term effects may be different to the short and medium-term effects. Needs to be studied over longevity of longest lived component species.

The impacts of scallop dredging were examined through an experimental study at three sites in southeastern Australia (Dromana, St Leonards and Portarlington), which were closed to scallop dredging in 1991. The areas chosen represented a wide range of sediment types in the 10-20m depth zone (depth at which most of the commercial scalloping takes place). Two experimental plots were located in each area. Each plot was dredged for a maximum of 3hrs/day for 2-4 days by a fleet of 5-7 commercial scallop vessel using 3m wide ‘Peninsula’ dredges. Number of scallops and by-catch species caught were recorded. To assess visual changes caused by the scallop dredge diver-operated video recorded was also carried out at each plot before and after the dredging at various time intervals (at one site Portarlington the site was not videoed before dredging due to poor visibility).

Habitat Effects

The dredging that occurred flatten all plots, however changes were most apparent at the St Leonards site which was dominated by callianassid mounds before dredging.

Species and Community Effects

Differences in by-catches species were clearly visible between the different areas. At Dromana the most abundant by-catch species in the sandy sediment were oysters, sea quirts, whelks, hermit crabs and giant spider crabs. In the muddy sediment at Portarlington sea squirts, sea cucumbers and brittle stars were the most abundant species, the by-catch at St Leonards included species from both of the other two sites. Dredges were most efficient on soft, muddy, flat sediment catching 51-56% of commercial sized scallops, where as on firm, sandy sediments with varying topography dredges were less efficient catching only 38-41% of commercial sized scallops.

kittiwakes made fewer feeding attempts in flocks that had greater numbers of gulls. Although kittiwake success rate per feeding attempt did not change as the number of gulls increased. Kittiwakes were more likely to avoid flocks that had a greater number of Glaucous-winged Gulls. Gulls successfully pirated less than one percent of fish captured by kittiwakes. The author's findings suggest that passive interference may be costly for smaller birds that feed in multispecies feeding flocks.

Location

Prince William Sound, Alaska

Reviewed by

Sewell et al 2007

Full Reference

Black, K.D., 1996. Aquaculture and sea lochs. , ,

Description

Symposium report with papers dealing with the physical environment, input of nutrients and chemicals, benthic enrichment, interactions between sea trout and other fish species, seabirds and mammals and aquaculture, the use of wrasse, the consequences of nitrogen enrichment and the possible effects of escapees on wild fish.

Several strategies to reduce the bycatch of seabirds in a salmon drift gillnet fishery in Puget Sound were examined, with the aim of reducing bycatch without causing a decline in fishing efficiency or causing an increase in bycatch of other species. Eight fishing vessels were contacted and divided into two groups; four experimental net types (i. traditional monofilament net, ii. visual alerts, 20 mesh, iii. Visual alert, 50 mesh, and iv. Acoustic alerts, pingers) were rotated around the four vessels each week. Each team completed 17 trips (in total 642 sets) during the experimental fishing. For seabird observations experienced observers on board the vessels recorded marine mammal and seabird abundance and entanglement per set as well as a range of physical variables (i.e. time, tide, visibility etc.)

Habitat Effects

Species and Community Effects

During the experimental fishing the target species sockeye was caught in 71% of the sets and seabirds in 25%. The capture of both sockeye and the entanglement of seabirds varied significantly with the gear type used, but were greatest in the monofilament nets. The 50 mesh nets caught the lowest numbers of sockeye and also entangled the lowest number of auklets when compared to the other methods. For the common murres all modified nets entangled a significantly lower number than the monofilament nets. The effect on marine mammals was also considered; pinger nets attracted a significantly greater number of seals than any of the other three methods. The affect of this modified gear had no effect on the incidental catch of other species in the fishery (e.g. dogfish). When compared with the monofilament nets the 50 mesh reduced common murre bycatch by 40% and the 20 mesh by 45%. For the auklet bycatch was only reduced by the 50 mesh (42%). As a result of this study three complementary tools to enable seabird bycatch to be reduced in the Puget Sound were identified: gear modification, abundance-based fishery openings and time-of-day restrictions.

Experimental investigation into changes in sediment structure, in- and epifauna, mortality of by-catch and effects on predators caused by beam trawling with the application of twice-yearly fishing perturbations.

Habitat Effects

Species and Community Effects

Trawling causes changes in the abundance of some in- and epifaunal species. Infaunal diversity reduced by 54 percent, epifaunal diversity not significantly altered. Mortality of animals retained in the cod-end studied by placing them in tanks. Results varied greatly between taxa. Mortality greatest for fish and animals with brittle skeletal structure such as sea urchins and swimming crabs, and very low for starfish, brittlestars and hermit crabs. Benthic species which are most likely to benefit from the increased scavenging opportunities brought about by trawling were starfish and hermit crabs.

Location

Red Wharf Bay and Dulas Bay in Liverpool Bay

Fishing Types

Beam trawl.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

House of Commons Environment, Food and Rural Affairs Committee, 2004. Caught in the net: by-catch of dolphins and porpoises off the UK coast 3rd Report of session 2003 - 2004. The House of Commons, London.. HC 88

Description

Investigation into by-catch of small cetaceans by the fishing industry operating from the UK coast.

Habitat Effects

Species and Community Effects

The harbour porpoise is particularly susceptable to being caught in bottom-set gill nets due to its benthic foraging behaviour. An independent observer on board Celtic Sea gillnet vessels between 1992 and 1994 estimated that vessels in the 15 m length and over sector took around 740 harbour porpoises per year during this period. A similar study of gill and tangle net fisheries in the North Sea comencing in 1994 estimated UK vessels took approximately 1000 porpoises during 1995 and 600 in 2000. Another previous study, reviewed in this report estimates that 200 common dolphins are also taken in the Celtic Sea gill net fishery per year. The report also mentions a large level of common dolphin bycatch from the bass pair-trawl fishery particularly during late February and March. Between 2001 and 2003, the average number of dolphins per trawl was four, with a maximum of ten in one trawl.

Review of direct and indirect threats of fisheries to seabirds, based on existing literature. For the purpose of this review, only issues relevant to species and fishing types used in and around the UK are summarised here.

Habitat Effects

Species and Community Effects

Long-lines Due to their feeding behaviour, most surface scavanging sea birds are pre-adapted to follow fishing vessels, feeding on discarded material and stealing bait from hooks. Birds will therefore often become hooked on longlines as the hooks are thrown overboard and birds will drown as the line sinks. Seabird mortality may be less if lines are set at night.

Gillnets In the north west Atlantic, a number of species of diving birds, also found in the UK are caught in high numbers by gill nets, while they hunt large shoals of small fish. In Greenland, large numbers of guillemot have been recorded as caught by salmon drift net fisheries. Gillnets set for bass in St Ives Bay, Cornwall have taken an annual by-catch of hundreds, possibly thousands of razorbills and guillemot. Studies around Wales have shown 'hot spots' of bycatch around bird colonies.

A review of non-native marine species found in British waters, including detailed information on each species. The report indicates that the greatest single source of non-native species in British waters (31.4 percent) have been by associated unintentional introduction with mariculture. 7.8 percent have been introduced by deliberate commercial introduction. Only species introduced by these methods (relevant to this report) and their effects on relevant species and habitats are described.

Habitat Effects

Species and Community Effects

It has been suggested that the phytoplankton species Coscinodiscus wailesii may have been introduced to UK waters from the Indian and Pacific oceans with imported oysters. When large numbers are reached, skeletons and minerals can 'blanket the seabed.

The red algal species, Asparagopsis armata, Bonemaisonia hamifera, Grateloupia doryphora, Grateloupia filicina, Agardhiella subulata and Antithamnionella spirographidis, also the brown algae Colpomenia peregrina are all thought to have been possibly introduced to European waters unintentionally with shellfish (most often oysters). The impacts, they are likely to have on the environment are however unknown. Another red algae Polysiphonia harveyi is known to have been introduced with oysters. It grows quickly on hard substrates and may displace native species. The brown algae Undaria pinnatifida may cause the displacement of native species on hard substrates and japweed Sargassum muticumis known to cause the displacement of native species including eelgrass and rockpool species. Both species are thought to have been introduced with non-native oysters. The green algae Codium fragile is thought to have been introduced with shellfish and displaces the native species Codium tomentosum. The gastropod Crepidula fornicata was introduced with the American oyster. It competes with native, filter feeding invertebrates for food and space and encourages the deposition of mud, rendering the substrate unsuitable for the settling of spat oysters. The American oyster drilll Urosalpinx cenerea was also introduced with American oysters. It predates native oysters and can consume up to 40 oyster spat per year. Two species of non-native oyster have been deliberately introduced to British waters Crassostrea gigas and Tiostrea lutaria, however neither are thought to have had a significant environmental impact. In the USA, C. gigas is known to have settled in dense agregations and displace native species. The American Hard-shelled clam Mercenaria mercenaria was deliberately introduced and now, fishing for the species can have a negative impact on seagrass beds. It is also likely that the presence of this species prevented the reestablishment of the native species Mya following a die of caused by cold weather.

Study on ecological effects of Manila clam cultivation at the end of the cultivation phase (for all stages see Gubbay and Knapman Reference 64)

Habitat Effects

Organic enrichment in net covered area. Short term sedimentation rates were up to 4 times higher in netted plots than control areas. The increase was localised. Increased organic matter, percentage fines and phaeopigment in the sediment and reduced water flow on the netted plots is likely to have had a major influence on the changes in abundance of some infauna species.

Species and Community Effects

Netting encouraged settlement of green macro-algae and in turn Littorina littorea. In the first 6 months fauna dominated by opportunistic species P.elegans. After 1 year the stabilising effect of netting and sedimentation led to establishment of species such as Ampharete acutifrons and Tubificoides benedii.

Paper looks at the results from surveys conducted by divers to estimate the dredge efficiency of the Northeast Fisheries Center (NEFC) 1.2m hydraulic clam dredge and to assess the effect that dredging had on the bottom substrate and fauna.

Habitat Effects

During the tow a cloud of silt (0.5-1.5m in height) was created, which settled within four minutes of the tow leaving a 75mm thick layer of silt over the fine to medium sand in the study area. The dredge track was clearly visible with a flat floor and smooth, sharply angled walls (depth of the track remained relatively constant at 23cm). During the tow sediment was pushed off to the sides by the dredge, these mounds of sediment are referred to as the track shoulder. The track shoulders were initially 15-35cm wide and 5-15cm high when measured against the undisturbed sediment. After 2 hours the walls had began to slump creating a rounded depression in the seafloor, the track shoulders had also begun to decrease (10-25cm wide, 5-10cm high). After 24 hours the track shoulders were no longer visible and the dredged path looked like a series of shallow depressions, it blended in with the general bottom features and was difficult to recognize.

Species and Community Effects

A negative effect on the target species (clams) was mortality seen as a result of cut clams or crushed clams. The cut clams were observed by divers whenever the dredge blade did not penetrate into the seabed by at least 20cm, large clams that burrowed deep into the sediment suffered the greatest amount of damage. A number of crushed clams were found after the dredge had been filled, mortality as a result of crushing was highest amongst larger clams. Predation was another cause of mortality to the target species. The greatest numbers of predators occurred inside the dredge track and were divided into two groups depending on whether they fed on the remains of damaged clams or preyed on undamaged clams.

Differences in benthic community structure and habitat complexes in areas exposed to different levels of bottom-fishing activity were assessed. The level of the bottom-fishing activity was dependant upon the restrictions imposed by a voluntary management agreements existing between towed bottom gear fishers and fixed gear fishers. Fishing effort was divided into three groups: 1) low fishing effort with 3 sites, one with no trawling permitted and two where potting only occurred all year, 2) medium fishing effort with 2 sites both with seasonal trawling and 3) high fishing effort with 3 sites where trawling was permitted all year. At each site a grid of 9 sampling stations was established, at each station infaunal samples (using an anchor dredge), epifaunal samples (using a 2m beam trawl), sediment samples (using a Day gab) and acoustic ground-discrimination data was collected. Five environmental parameters were quantified: median grain size, water depth, percentage organic content, mass of stone and mass of shell.

Habitat Effects

The video-transect showed that the rocky outcrops and reefs occurred throughout the closed areas it is known these reefs can support a number of species. The reefs consist of fragile rock that can get caught when sampling gear is misplaced or when the voluntary agreement is broken. The number of stones and rock fragments in the survey samples were much higher in areas that had been exposed to towed fishing gear.

Species and Community Effects

In the areas closed to fishing activities communities were dominated by higher biomass and emergent fauna that increased the habitat complexity. One species in particular was most abundant in this closed area, Glycymeris glycymeris, this species is vulnerable to fishing because it lives close to the sediment surface and reproduces infrequently. In comparison those areas where towed gear was permitted were dominated by scavenging taxa and smaller-bodied fauna.

Study examining differences in seabird and wader community composition in an area of oyster cultivation, compared to a reference area with no oyster cultivation. Trestles measuring 40cm high, 90cm wide and 3m long were used for cultivation in an area of one hectare. Of this, an area of 4500 m2 of trestles was covered with oyster bags. Observations of bird behaviour and counts were carried out.

Habitat Effects

Species and Community Effects

All species observed in the study were seen at both sites. The outcome of the study indicates that oyster structures did not effect the feeding behaviour of the birds and the six species with the most data available did not appear to be affected by the trestles. However, the number of birds overall in trestle areas was lower than reference area (except for redshank and dunlin). Some wildfowl species such as the wigeon fed on green algae, growing on trestle tables, only when the water around the table was deep enough to swim, but not covering the tables. The authors mention that elswhere, Brent geese have been observed displaying similar behaviour. The authors observe that feeding rate is generally higher in days with shorter tidal exposure.

The sea scallop fishery has been managed under the Sea Scallop Fishery Management Plan (SSFMP) since 1982, with measures initially focusing on controlling the age at entry into the fishery in an effort to maximize yield per recruit. This method proved to be inadequate and the populations continued to be overexploited, with high levels of capture and mortality of small sea scallops. In 1994 Amendment 4 of the SSFMP was adopted in an attempted to reduce fishing mortality by 70% over a 7 year period. The changes to the gear in theory would allow juvenile sea scallops to escape instead on relying on crew to discard them. The studied assessed the size selectivity and relative efficiency of sea scallop dredges and trawls under Amendment 4, to determine if the gear restrictions of Amendment 4 were effective at controlling the sea scallop age at entry into the fishery. Three fishing cruises were conducted using the trawl and dredge gear by utilizing a parallel fishing method, where the two vessels fished the same ground at the same time and sampled from a single sea scallop population.

Habitat Effects

Species and Community Effects

Target species: The results indicated that there was no significant difference between the relative harvest efficiency of the two gear types for shell height between 85 and 95mm. The trawl vessels harvested sea scallops more efficiently when the shell height was less than 85 to 95mm, where as the dredge vessels harvested sea scallops more efficiently when the shell height was more than 85 to 95mm. The abundance of sea scallops caught between the two gear types varied, the differences occurred due to differences in the shell height of the sea scallop. During the May 1998 trip the trawl vessel caught 35.4% more scallops per hectare than the dredge vessels, this was due to the presence of a large number of 70 to 90mm shell height sea scallops.

Report aimed to review the current management arrangement for inshore fisheries in Scotland considering the need for environmental protection and make any recommendations necessary to improve the management system.

Report looks at the impacts that eider ducks have on mussel farms in Scotland, particularly as the eider ducks are consuming mussels from cultivated lines. Various methods that could be used to reduce the impact are discussed.

Habitat Effects

Species and Community Effects

Within the UK the population of eider ducks has increased by about 2.5% each year (population size would double every 30 years), however in Shetland the population is on a downward trend of about 4% per year and has been for the past 20 years for which the reason is unknown. Recent surveys that have been conducted in the Argyll and the Clyde (west Scotland) on eider distribution have shown a strong association between mariculture and local concentrations of eiders. Mariculture has influenced the local distribution of eiders in west Scotland at particular times of year. Results from a survey conducted in September 1998 showed that of the 558 eiders in Mull, 380 were on large mussel farms in Loch Scridian, 105 were around salmon cages in Loch Spelve and 25 were on other mussel and fish farms. Only 18 were found on the remaining coastline of the Island (away from aquaculture systems). Between 1998 and 1999 there was a decrease in the number of eiders on Mull, which happened to not only coincide with the closure of a salmon farm, but also that in 1998 mussel farmers lost 90% of their stock to eiders so carried out a number of deterrent methods to reduce their losses in 1999.

Survey into the effects of predator control measures around aquaculture facilities.

Habitat Effects

Species and Community Effects

Grey seals, common seals, cormorants, shag and mink were the most prevalent predators with most of the fish farms surveyed suffering losses to some or all of them. Eider duck and, on some occasions oyster catchers are known to feed on shellfish farms. Predator control measures can be detrimental to all these species which can get tangled and drown in predator nets. Tangling in fish farm nets, mostly top nets and predator nets, was reported from 68 percent of the 47 sites visited. The animals reported caught were seals, herons, cormorants, shags but also gulls, eider duck, black guillemot, great northern diver, gannet, dolphins (unspecified), harbour porpoise and even a basking shark. Seals, herons, cormorants and shags have also been shot by fish farm operators to protect the stock.

The main impacts of predator control around fish farms are disturbance, displacement and killing both directly and indirectly. More detailed information is needed to assess the significance to local populations but author suggests that it is likely to be acute given the concentration of destructive control measures around individual farms.

The study was divided into three sections. The first examined the effects of Nephrops creels in a Scottish sea loch, on sea pens. The second examined the immediate effects of hauling pots on the benthos of different habitats, ranging from exposed limestone slabs, to large boulders and rocks interspersed with gravel. The third was a quantitative study into the effects of crab and lobster pots on benthic species on rocky substrata.

Habitat Effects

Species and Community Effects

Sea pens Pennatula phosphorea, Virgularia mirabilis and Funiculina quadrangularis were able to recover from all creel impacts, by bending to avoid the impact of dropped creels and reinserting themselves following uprooting. During observations of pot hauling over rocky substrates, the pink sea fan Eunicella verrucosa was observed to bend under the weight of the pot, returning to an upright position afterwards. Quantitative studies revealed that there were few immediate detrimental effects resulting from four weeks of intensive potting over rocky substrata. Damage was however inflicted on large, slow growing ross 'corals' Pentapora foliacea [now Pentapora fascialis] by pots.

Interim report giving some results of a five year project, examining the ecological effects of some medicines used to control sealice in salmon aquaculture. Aspects measured include settlement of flora and fauna on settlement panels, studies of meiofauna and macrofauna in sediments, phytoplankton and zooplankton sampling and analysis.

Habitat Effects

Species and Community Effects

Although the analysis of samples is not yet complete, preliminary findings indicate that there has not been any catastrophic perturbation of the sea lochs studied. This indicates that if these medicines have ecosystem effects they are either difficult to separate from the natural variability present in such systems or are below the limits of detection of the methods currently available.

Location

Scottish sea lochs

Fishing Types

Mariculture (finfish).

Reviewed by

Sewell & Hiscock 2005

Full Reference

Thompson, P.M., 1992. The conservation of marine mammals in Scottish waters. Proceedings of the Royal Society of Edinburgh, , 123-140

Description

Review paper

Habitat Effects

Species and Community Effects

Seals are still killed around the Scottish coast where they interact with fishing or fish farming interests but it is difficult to assess the impact. Probably localised and limited in extent, but could have a significant effect on some local populations. Seals and cetaceans may be caught accidentally in fishing gear and anti-predator nets around fish farms. Grey and common seals, harbour porpoises and common dolphins are the most commonly caught species in UK waters. Currently the assessment of the significance of the potential threats is hampered by lack of data on the nature of the threats and the dynamics of the populations concerned.

Study was a meta-analysis of 101 different fishing impact manipulations, for this 101 different experimental manipulations /observations were examined for the effects of fishing disturbance on benthic fauna and communities. To classify the experimental studies a number of different variables were considered that might affect the degree of fishing impact, these included: fishing gear type, water depth (m), disturbance regime, habitat type (e.g. mud, sand, biogenic habitat), taxonomic grouping (e.g. by phylum) and minimum dimension of the reported scale of disturbance (e.g. width of trawl). Studies that also considered recovery rates were also important.

Habitat Effects

Intertidal habitats: the impact of intertidal dredging was shown to be much more severe than that of intertidal raking. With intertidal raking the sediment was left in situ, only the upper few cm were disturbed by the gear, however intertidal dredging resulted in the removal and resuspension of sediment in the water column.
Subtidal habitats: Both beam trawling and scallop dredging had short-term negative impacts on muddy-sand and sand habitats. Otter trawling had a significant initial impact on mud and muddy-sand habitats, but the effects appeared to be short and the long-term outcome was positive. The studied showed that otter trawling on biogenic habitats caused negative impacts, but these were not as severe as those caused by scallop dredging. The scallop dredging on biogenic habitats gave the greatest initial negative impact and these effects were predicted to last from 972 to 1175 days after the dredging stopped.

Species and Community Effects

Both deposit and suspension feeders were vulnerable to scallop dredging across gravel, sand and mud habitats. For recovery, slow growing large-biomass biota (e.g. sponges and soft corals) took much longer to recover from the effects of fishing, up to 8 years. Biota with short life-spans (e.g. polychaetes) recovered much quicker, less than 1 year. Biota of soft-sediment habitats, especially muddy sands, also had recovery periods predicted to be in years.

A meta-analysis of 39 published fishing impact studies was undertaken, with the aim of identifying whether there was sufficient data on the impacts of fishing activities on benthos to answer the following questions: 1. Are there consistent patterns in the responses of benthic organisms to fishing disturbance? 2. How does the magnitude of this response vary with habitat, death, disturbance type and among taxa? 3. How does the recovery rate of organisms vary with these factors? Any gaps in the data were also identified.

Habitat Effects

The largest negative impact occurred on biogenic reef and muddy sand and gravel habitats. The recovery of habitats was quickest within sand habitats, which tended to be less physically stable and also contained more opportunistic species, as opposed to any other habitat type.

Species and Community Effects

Within the disturbed plots the total number of individuals decreased by 46% and the total number of species decreased by 27%. Results also indicated that inter-tidal dredging and scallop dredging had the largest initial impact on benthic biota and trawling had the least impact.

The study uses a behaviour based model to assess the body condition and mortality of three shorebirds, i) curlew Numenius arquata, ii) dunlin Calidris alpina and iii) oystercatcher Haematopus ostralegus which may result due to an extension of the port at Le Harve and the potential mitigation measures put in place.

Habitat Effects

With a 20% reduction in habitat the body condition and mortality of the curlew was unaffected, however there was significant decrease in the body condition and increase in mortality in both the oystercatcher and dunlin.

Species and Community Effects

Two mitigation measures were assessed, i) the introduction of a buffer zone in order to reduce disturbance to feeding birds and ii) the introduction of a new mudflat in order reduce the effects of habitat loss. The introduction of a buffer zone restored both body condition and mortality to pre-disturbance levels across all three species of shorebird. Only the dunlin and oystercatcher were affected by a reduction in habitat loss. In order to restore body condition and mortality of the oystercatcher to baseline levels a 50 ha mitigation area of the same quality as the adjacent mudflat would be required. In order to restore body condition and mortality of the dunlin to baseline levels a 100 ha mitigation area of the same quality as the adjacent mudflat would be required.

Study aims to determine if the ranging patterns of bottlenose dolphins have been altered by oyster farming in Shark Bay. To answer this three questions were addressed: i) do bottlenose dolphins change their use of an area once farming begins; ii) do bottlenose dolphins move away from the farm; iii) do bottlenose dolphins move around (not through) the farm?

Habitat Effects

Species and Community Effects

Study indicated that bottlenose dolphins have been displaced by aquaculture. There was a significant decrease in the use of the extension area when the oyster farming was introduced. The strongest evidence came from the movement around the oyster farms, when compared to ecologically similar areas nearby, the adult females stayed to the outside of the farm rather than going through the pearling lines. Displacement also has the potential to affect foraging capabilities and reproductive success.

the authors conclude that the level of oyster culture (8 kg m-2) currently practiced is not sufficient to negatively impact either the sediment biochemistry or the associated benthic community. But suggest continued environmental monitoring is important to ensure that the overall health of the ecosystem is maintained.

Pre-dredging surface followed by qualitative and quantitative assessments (although not at the same stations), photographs and sediment samples.

Habitat Effects

Conspicuous tracks on the seabed about 4m wide. At each site a ridge of stones, shells and shell fragments approx. 15cm high and 30cm wide. Inside ridges shallow grooves formed by rubber bobbins at the ends of the towing beam. Examination of tubes of the anemone Cerianthus lloydii in the dredge paths suggested top 2-4cm had been removed. Passage of dredge created a thick sediment cloud the heaviest constituents of which settle out rapidly and close by. Fine sediments were carried away by the tide.

Species and Community Effects

Dredge bags contained shells and stones most of which supported sponges, hydroids, small anemones, tube-worms, barnacles, ascidians and bryozoans. Remains of several P. folicacea and large numbers of small crustaceans (chiefly Pilumnus hirtellus), molluscs (especially Trivia spp.) and juvenile echinoderms within the folds of the colonies. Also several sponges (mostly Suberitesspp.) and a large number of epibenthic echinoderm species in the catch. Predators and tidal currents removed much evidence of killed or injured animals in the 24 hours after dredging but dead or damaged tubeworms, crabs, squat lobsters echinoderms and P. foliacea were found. Large numbers of C. lloydii present in dredge path. Broken tops of l. conchilega tubes were common in dredge paths but large numbers of intact tubes suggested that the worms had survived and rebuilt their tubes. Large mobile epifauna generally absent from dredge path except for occasional scavenging A. rubens although within 48hrs smaller mobile species such as hermit crabs were present. Counts of infauna in and immediately alongside dredge paths showed these species were unaffected by the level of dredging. Sessile species found during presurvey but not seen in dredge paths include shell fauna, C. celata, Suberities spp. A. digitatum and P. foliacea.

Study quantifying the mortality and numbers of animals caught by a fleet of 12 crustacean pots, left of a rocky seabed in "a manner designed to simulate ghost-fishing". Information on the pots was gathered by diver surveys at, 1, 4, 12, 27, 40, 69, 88, 101, 125, 270, 333, 369, and 398 days after initial deployment.

Habitat Effects

Species and Community Effects

During the experiment, seven species were captured in the pots. Crustacean species caught were spider crabs Maja squinado, brown crabs Cancer pagurus, velvet swimming crabs Necora puber and lobsters Homarus gammarus. Fish caught in traps were ballan wrasse Labrus bergylta, trigger fish Ballistes carolinensis and lesser-spotted dogfish Scyliorhinus canicula. Spider crabs were captured at a mean catch rate of 7.08 per year and edible crabs at a mean catch rate of 6.06 per year and were the most common species to be caught. The lesser-spotted dogfish and triggerfish were only caught on one occasion each during the study, equating to a mean catch rate of 0.08 per year for these species.

A draft management plan to support regulations that will ensure the sustainable harvesting of cockles and mussels from the Solway Firth. The report includes a review of the potential impacts of various methods of exploitation of these species. Boat dredging was banned in 1992 and tractor dredging banned in 1994, following this, hand gathering increased in intensity and this too was prohibited in 2002.

Habitat Effects

Species and Community Effects

Following increased catches of cockles that accompanied the introduction of suction dredgers into the Solway Firth there was extremely poor recruitment of cockles and the fishery began to decline. Up to 10 percent of hand gathered cockles may be damaged, but the extent to which this occurs depends on the expertise of the gatherer.These rates are likely to be higher for mechanical harvesting techniques. The use of an 'elevator dredge' can vastly increase the survival rate of cockles, including undersized specimens, which can be re-seeded in areas of low spatfall or where they are likely to grow faster.The infauna at exposed sites is less effected by disturbance related to cockle dredging than at undisturbed sites. Hydraulic dredging can potentially result in the complete disapearance of Zostera marina beds. All terrain vehicles used by hand gatherers can be extremely damaging in intertidal areas. Particulary if used over Zostera beds. Wildfowl can be affected by competition for food (cockles) and by disturbance. Removal of the food source may result in mass mortalities in some species. Fishing method, which require fishers to be present at low water can be significantly more damaging to bird populations that those that take place at high water.

Experimental dredges were carried out by a commercial water jet, dredging vessel, targeting razor clams (Ensis spp.) fishing was conducted for 10min periods. 6 tracks were intensively studied by divers, who observed sites during dredging, collected cores for analysis, made measurements of physical impacts and made observations of epifauna. Video footage of trawls sites was also used.

Habitat Effects

A flat-bottomed 'V' shaped Trench was left, measuring 1.2 m surface width, 0.5 m base width and 0.15 m depth. Track lengths varied between 26 and 122 m. Tracks were still visible but less pronounced after five days. After 11 weeks tracks were no longer visible. However, sediment was still in a state of fluidisation to a depth of 0.2 m.

Species and Community Effects

Some short term changes were recorded between infaunal assemblages immediately after and five days after the trawl, but no difference was apparent after 11weeks. Within a day of fishing, the number of infaunal species and number of individuals within trawl tracks had significantly decreased, but no difference was recorded after five days. A reduced biomass in fished areas, compared to control sites was still evident after five days. None of the diversity parameters studied showed significant effects of fishing. Due to the mobile nature of the sediment, epifauna was limited, but there was an increase in large scavangers in trawled areas imediately after each trawl. Several larger organisms where captured as bycatch and many of these showed signs of damage.

Study of Bottle-nosed dolphin strandings, including examination of cause of death.

Habitat Effects

Species and Community Effects

153 individuals were examined. sixteen showed signs of fishery related trauma. Five males and eight females were invloved in net entanglements.The majority of these had stomach contents containing shrimp or fish remains, indicating interactions with commercial trawl fisheries. The majority of interactions took place during the summer months.

Study examining benthic species assemblages, subjected to four different types of commercial fishing pressure. These were: i) Towed gears only, ii) annual, seasonal towed-gear use, iii) temporary towed-gear use but reverting to static gear use 18 - 24 months before sampling, and iv) static gears only. The survey was undertaken in an IPA (Inshore potting agreement) area, where towed gears had been previously banned, but potting was allowed. Video surveys were used, combined with sampling with towed dredges.

Habitat Effects

Species and Community Effects

Higher biomass and diversity of species was found in sites that had not been trawled in the year prior to sampling, compared to towed gear sites. Untrawled areas had higher biomass, but lower species diversity than 'ex-trawl sites'. The Authors suggest that the most important finding of the study was that very little difference existed between benthic communities in trawled sites and seasonally trawled sites. It was suggested that this indicated that a six month cessation of trawling is insufficient to allow recover of benthic communities. Significantly greater biomass of attached species were found at untrawled sites than all other sites. The authors note that this is important, as many attached species are known to provide settlement sites for other benthic species and shelter for a number of fish species.

For full details of this study, see also: Bergman, M.J.N. & van Santbrink, J.W. 2000. Mortality in megafaunal benthic population caused by trawl fisheries on the Dutch continental shelf in the North Sea.

Study to calculate direct mortality of infaunal and epifaunal species of invertebrates, following otter and beam trawls. The annual fishing mortality for megafaunal invertebrate populations in the Dutch sector of the North Sea was also estimated based on the results of this field study. Three types of commercial beam trawls were tested (12 m wide and 4 m wide with tickler chains and 4 m wide with chain matrices) as well as an otter trawl with a 20 m net width. Mortality was determined by measuring species density before and comparing this with density 12-24 hours after trawling.

Habitat Effects

Species and Community Effects

Single tows of 4 m and 12 m beam trawls, resulted in direct mortality of a number of species, ranging from 5 percent to 50 percent and up to 68 percent for some bivalve species. There were lower levels of mortality associated with otter trawls than beam trawls of all sizes. Mortaliy of organisms was greater in silty sediment than sandy sediment. Most direct mortality took place either as a result of impact by the trawl or disturbance and exposure leading to predation. Only a relatively small percentage of mortality was due to organisms being caught in trawls and discarded.

The aims of the current study were to:
1) Identify of the key wintering areas used by the white-chinned petrels from South Georgia.
2) Extend of sample tracks that are collected during the winter.
3) Assess the individual variability in site preference.
4) Determine what oceanographic factors influence site selection.
5) Quantify spatio-temporal overlap with fisheries – focus on conservation and management.
The Global Location Sensors (GLS-I loggers) were attached to plastic leg rings and deployed on the tarsi of 35 adult white-chinned petrels, each bird was taken from a different burrow and the burrow marked with a wooden stake (burrows visited 9 times during the incubation period in the following 2 years and at the end of the study to remove the devices). The key wintering areas were identified by generating kernel density maps, white-chinned petrels at South Georgia lay between November 13th and December 10th, the location of the tagged birds indicated that each made a trip to the Patagonian Shelf right before this period. In order to assess areas of high and low utilization during the winter period, kernel density contours were laid over maps of bathymetry and monthly sea surface characteristics were recorded. The distribution of the tracked white-chinned petrels was also compared with number of reported hooks set by major longline fisheries operating in overlapping and adjacent fishing grounds.

Habitat Effects

Species and Community Effects

Ten birds were tracked for 266-664 days (mean 366), all of which migrated to the Patagonian Shelf and shelf-break waters. Several major fisheries overlapped with the distribution of white-chinned petrels, many of these fisheries are suspected to have high seabird by-catch rates. This is an area which needs to be addresses and although closed areas and seasons may not be viable solution encouraging well-regulated licensing procedures may be an option. An economic incentive for fishermen to use mitigation methods would be in order to reduce bait loss when line setting as white-chinned petrels frequently dive for the bait. Measures need to be introduced otherwise the outlook is bleak.

Experimental study using an area closed to scallop dredgers since 1989. Experimental plots were set-up outside the closed area, in an area still exposed to commercial trawling, unfished plots and experimentally trawled plots were also set up inside the closed area. Plots were studied using grab sampling and diver counts of Pecten maximus.

Habitat Effects

Species and Community Effects

Benthic communities in experimentally dredged plots became less similar to adjacent undredged sites and more like commercially dredged sites. Since 1989, an increase in numbers of and age of Pecten maximus occurred in the closed area.

This report presents the findings from a 2 year project, studying the distribution, biology and ecology of shad (Alosa sp) in the Environment Agency's south-west region. The project includes the collection and analysis of various reports of recreational and commercial fishery captures of shads, at sea and from estuaries.

Habitat Effects

Species and Community Effects

Since 1970, in the south west of England, catches of both UK species of shad have been recorded from a number of different fisheries. The most common capture method for shad was trawling, particularly during the winter. Shad catches were recorded by pair, otter and beam trawls, bass nets, drift nets, salmon nets, seine nets and by rod and line. During the summer months, the majority of recorded shad were caught on rod and line from the shore, in estuaries or coastal waters.

Study using reports of incidental catch of harbour porpoise. Most are killed in monafilament gill nets set for groundfish or pelagic species.

Habitat Effects

Species and Community Effects

Estimated total catch for the year in the area (based on notifications by fishermen) was 105+10.8 animals. The animals were entangled while nets on the bottom in water depths of 37-96m. They seem to catch certain size classes and not small or large animals. Factors other than fishing effort may also have effected the incidental catch rate of harbour porpoise. In one area it was disproportionately high, perhaps reflecting the high density of porpoises in the region.

There were no changes in porpoise density in the region between 1980-86, but two significant changes in length frequencies (increase in length of calves and absence of large porpoises in the recent samples). These changes may be attributed to the fishery which has been operating for 10-15 years. The effects of sustained adult mortality in the gill-net fishery appear to have compressed the size and possibly the age structure of the population perhaps reducing the reproductive lifetime of females. Given the slow reproductive rate authors consider that these incidental catches seriously threaten the population as porpoises in Bay of Fundy and Gulf of Maine apparently form a relatively discrete population unit.

Beach based and aerial surveys were used to quantify gillnets and dolphin numbers in coastal waters. Strandings investigations were used to characterise interactions between bottlenose dolphins and monofilament, gill net fisheries.

Habitat Effects

Species and Community Effects

Highest numbers of dolphin mortalities resulting from gill-net interactions were recorded between October and November. During October 1997 four stranded dolphins had been killed as a result of interaction with gill-nets. One individual was captured alive and subsequently released from a gill net and in October and November 1998, six stranded individuals were found to have been killed by gill nets.

Pre and post experimental investigation, within 30m depth contour, with 7 tonne, 12m beam trawl including 5x22mm and 3x18mm tickler chains, 3x20mm and 8x14mm net tickler chains, mesh size of 9cm in the cod-end. Area trawled three times over 2 days and samples taken up to 2 weeks after trawling.

Habitat Effects

Tickler chains penetrate at least 6cm into the sediment surface indicated by catches of Echinocardium cordatum and Arctica islandica. Tracks made by the beam trawl shoes were still apparent on sidescan sonar after 16hrs.

Species and Community Effects

Some benthic species showed a 10-65 percent reduction in density after trawling the area three times. There was a significant lowering of densities (40-60 percent) of echinoderms Asterias rubens and small Echinocardium cordatum, and of polychaete worms Lanice conchilega and Spiophanes bombyx. Vertical distribution in sediment appears to be an important factor in catchability. Decrease in density (10-20 percent), although not significant for small crustaceans and larger Tellina fabula and E. cordatum. Except for the starfish A. rubens most of these animals live in the sediment at a depth up to 15cm. The effect of beam trawling on densities of small individuals tends to be much greater than on densities of large individuals (larger animals tend to live deeper or have better escape possibilities). The polychaete worm Magelona papillicornis showed a considerable increase in numbers, this may be attributable to a change in the vertical distribution of the species in the sediment. The numbers of small Ophiura living in the top centimetre of sediment did not change after trawling the area three times, suggesting the species escape unharmed through the net mesh. Also no direct effect on densities of molluscs (except T. fabula) and worms (except Magelona papillicornis, L. conchilega and S. bombyx). Less abundant worm species (including Spio filicornis, Scolelepsis bonnieri, Scoloplos armiger and Owenia fusiformis) and less abundant molluscs (including Thracia sp. Venus striatula, Montecuta ferruginosa and Mysella bidentata) showed no change in total density after trawling. About 90 percent of A. islandica caught by the 22m trawl were severely damaged.

Side scan sonar investigation into the effects of beam trawling in the southern part of the Danish North Sea.

Habitat Effects

Poorly preserved trawl marks were widely distributed in the study area except in one area of presumably coarse grained sediments where there were numerous extremely well-preserved beam trawl marks. The substrate appears to have altered from coarse grained sand or gravel to fine sand and coarse silt in the trawl marks as shallow scouring and smoothing from beam trawling created conditions favouring fine sand/coarse silt sediment filling the tracks. Effects of beam trawling on sediment may be long-term and in some areas may have resulted in a definitive change of the substrate with implications for the benthic community.

Species and Community Effects

Location

Southern North Sea

Fishing Types

Beam trawl.

Reviewed by

Gubbay and Knapman, 1999

Full Reference

Rijnsdorp, A.D., Buijs, A.M., Storbeck, F. & Visser, 1996. Micro-scale distribution of beam trawl effort in the southern North Sea between 1993 and 1996 in relation to the trawling frequency of the sea bed and the impact on benthic organisms ICES . C.M. 1996/Mini 11.

Description

Study into the micro-scale distribution of beam trawl effort in the southern North Sea between 1993 and 1996 in relation to the trawling frequency of the sea bed and the impact on benthic organisms.

90m long gill net (100mm diameter mesh) and trammel net (100mm with 600mm diameter outer mesh) set by commercial fisherman and cut at one end to simulate net loss. Survey of catches by direct observation, still and video photography for the following 9 months.

Habitat Effects

Species and Community Effects

Both nets caught large numbers of elasmobranchs which took about 3 weeks to decompose. Gadoids were eaten within 72hrs therefore not possible to tell how many were caught throughout the observation period and estimates were considered by authors to be conservative. Initially both nets caught more fishes than crustaceans but by 20 days crustacean catch was greater than fishes and was greatest 43 days after initial deployment. Catch per 24hr period declined with time and for fish was nearly zero at 70 days for gill net and 22 days for trammel net. Catch per 24hr for crustaceans remained higher than for fish for both nets throughout the study. Reduction of catch rate probably linked to reduction in net size and degree of entanglement. Overall catch over the 134 day experiment was 261 animals in the gill net and 292 in the trammel net.

Maja squinado and Scyliorhinus canicula were the 2 species most commonly caught in both nets. Other species caught were lobster, brown crab, swimming crabNurse hound and Smooth hound. All the crustaceans caught known to scavenge carrion. Other scavengers also aggregated to feed on the animals in the nets included A. rubens, M. glacialis, O. fragilis (in large swarms) and E. esculentus. Three shags were also caught. When nets retrieved (3 months after last survey) 2 spider crabs, previously marked were still alive after more than 102 days in the net. Towards the end of the experiment the free end of the nets began to roll up reducing the total length of net. Authors conclude that total catch of animals during life of a net may be considerable as in the present study but will depend on local fauna, habitat type and environmental conditions at the site.

Preliminary results of a dive survey to examine the status of Modiolus modiolus beds in Strangford Lough. One focus of the survey was to assess whether any recovery of the reefs had taken place since conservation measures to reduce fishing activity were introduced in 1993.

Habitat Effects

Species and Community Effects

The survey found no evidence to suggest recovery of the reefs since 1993. The authors conclude that the reefs are 'no longer in favourable conservation status' and that the use of bottom fished gear poses the most immediate threat to the few remaining clumped Modiolus beds within the Lough. In a site zoned for trawling for queenies that had previously contained a Modiolous with Chlamys biotope, no clumped Modiolus remained. Divers also observed very few queen scallops remaining in the area.

Location

Strangford Lough, Northern Ireland

Fishing Types

Various (Not listed).

Reviewed by

Sewell & Hiscock 2005

Full Reference

Service, M. & Magorrian, B.H., 1997. The extent and temporal variation of disturbance to epibenthic communities in Strangford Lough, Northern Ireland. Journal of the Marine Biological Association of the United Kingdom, 77, 1151-1164

Description

Video surveys (1990) and side scan sonar (1990 and 1993) were used to determine the impact of trawl fisheries on, and epibenthic community associated with Modiolus modiolus beds in Strangford Lough.

Habitat Effects

Scars made by otter trawl doors were clearly visible using side-scan sonar. Changes to epibenthos, including evidence of lost mussel beds (broken shells etc) were visible on video surveys. Only one scallop dredge scar was observed, on one occasion during the 1990 survey. Clear evidence was seen that trawling had altered the supperficial structure of sediments in some, heavily trawled areas. Between 1990 and 1993, no evidence was found of temporal change.

This study examined the potential effects of hydraulic dredging on maerl beds. A flourescent sediment tracer was used to mark dead maerl, that was laid over the area to be trawled. The maerl was laid on the seabed in a way, that represented the natural maerl bed typical of the area. Following the passage of the hydraulic dredge, dredge track observations, catch analysis, and assessment of maerl catch and sediment resuspension were carried out.

Habitat Effects

Large quantities of dead maerl were caught by the dredge. Only a relatively small proportion of dyed maerl was captured, as the majority was dragged along the dredge track and reburied. A large amount of fine sediment was resuspended by the trawl, when it settled, maerl around the dredged path was blanketed by newly settled silt. This blanketing effect was easily discernable at least 21 m away from the dredged path.

Species and Community Effects

A large number and high diversity of benthic organisms were captured in the dredge, including many large, long-lived, deep burying animals. Many larger, more fragile organisms were killed, whilst smaller more robust organisms were largely unharmed. Very few active species were captured, reflecting the slow speed of the dredge. Some live maerl thalli were also caught in the dredge.

Changes in sediment composition and benthic community structure under cultures studied over 3 years in a narrow sound, 13-15m deep with generally weak currents..

Habitat Effects

Faecal material and mussels drop to the seabed. As a consequence a layer of sediment was found to increase at a rate of 10cm/yr. This resulted in the production of H2S in the uppermost layers. Small grain size, high organic content and a negative Redox potential recorded under the cultures and changed with distance from the culture.

Paper looks at the effects that marine aquaculture has on benthic communities and the management measures in place to reduce these impacts and long-term monitoring programs. Marine farming is based on two main species: Pacific oyster Crassostrea gigas and Atlantic salmon Salmo salar. The effect of shellfish farming on benthic environments was investigated at three sites (all long established subtidal mussel and oyster farms) and one control site. Using physical and chemical measures sediment condition was assessed, video recording was conducted under the longlines and benthic invertebrate species composition and abundance were assessed.

Habitat Effects

Results indicated that shellfish faming within the lease area and no impacts on the area outside the lease boundary. Redox, sulphide levels, organic carbon and rates of decomposition did not vary significantly between the inside and outside of each farm.

Species and Community Effects

The composition of benthic infauna was not significantly different between the inside and outside of each farm; the differences that did occur were between the different farms.

Report of a long term study, looking at the process of cultivating and harvesting the intertidal manila clam Tapes philippinarum . The study began with the seeding of the clams, then through on-growing and harvesting 30 months later.

Habitat Effects

Species and Community Effects

Early studies revealed that the netting used to cover seeded clams encouraged the growth of certain deposit feeding polychaete species. However the immediate effects of harvesting by suction dredging caused a reduction of infaunal species and their abundance by approximately 80 percent. Based on comparison with undredged control plots, sediment structure and infaunal communities at netted and dredged and netted but undredged sites had recoverd within 12 months of harvesting.

Report of catches in the Solway. A major cause of mortality to otters has been accidental capture and drowning in fish and crustacean traps. Four types of guards for eel fyke nets were constructed and tested - square guard, ring guard, front net guard, grid guard. Effects on catches of eels (total weight, number and catch of saleable eels) were recorded. Techniques other than guards discussed but it was considered that the only safe and continually working otter protection device was a physical barrier at some point near the mouth of the fyke. The Steering Committee set up to look at the problem suggested authorities should consider and adopt most suitable designs for their situation and then consider ways of implementing and ensuring use.

Habitat Effects

Species and Community Effects

Otters investigate eel fyke nets because of the artificially concentrated prey in the cod end. They are unable to bite their way through modern multifilament nylon netting therefore the only way to get the prey is through the fyke entrance and down through the funnels. The time they can submerge is not sufficient in many cases for an otter to negotiate its way back to the entrance so it drowns. Between 1975-1984, 89 otters are known to have been caught in underwater traps (50, 33 and 6 in eel fyke nets, crustacean and fish nets). In the Solway verified data considered by an observer to be only 20-50 percent of the real total. Fish traps can be effective at reducing otter populations when set for a long period in a single locality.

Comparison of substratum heterogeneity of a dredged site (Stravanan Bay) and an undredged site (Caol Scotnish).

Habitat Effects

Structural heterogeneity was far lower in impacted, dead maerl, which had similar heterogeneity to gravel. Unimpacted maerl had higher structural heterogeneity.

Species and Community Effects

Location

The Stravanan Bay, Isle of Bute and The Caol Scotnish, Loch Sween. Scotland

Fishing Types

Scallop dredge.

Reviewed by

Sewell & Hiscock 2005

Full Reference

Rehfisch, M.M., Clark, N.A., Langston, R.H.W. & Greenwood, J.J.D., 1996. A guide to the provision for refuges for waders: an analysis of 30 years of ringing data from the Wash, England. Journal of Applied Ecology, 33, 673-687

Description

The aim of the study was to determine how far apart roost refuges should be in order to benefit populations of wading birds. From 1959 to 1993 the Wash Waders Ringing Group collected the largest set of data on ringed waders from any site worldwide. Catches were made every month, although were more frequent from July to August, by cannon-netting or mist netting. Birds were ringed at 85 roost sites around the Wash. By analysing the movements of the waders between the roosts it would provide a way to determine the mobility of the different species and how the spacing of refuges may effect the normal roost movement. A model was used to describe the wader dispersal between roosts and then used to estimate the effect of inter-refuge distance on the number of waders reaching at least one refuge during normal roost movements. This may implicate how refuges are created and managed.

Habitat Effects

Species and Community Effects

Despite the fact that waders fly long distances during migration, the study indicated that once at their wintering ground they only travel short distances. The overall outcome indicated that 50% of the populations would be catered for if refuges were 7-10km apart, however this would increase to 75% of the populations if the refuges were 3-6km apart.

The main aims of the project were to firstly review the current design and operation of razorfish dredges in other European fisheries and identify a suitable design for The Wash. Secondly, field experiments were conducted in The Wash using an appropriate dredge to assess the impact of the dredge on the interest features and evaluate how efficient the dredge was at collecting the razorfish and assess the damage rate of the razorfish. To assess the damage to the seabed two sites were tested with a different pressure level at each site; i) Seal Sand, pressure level increased slowly with approximately 1 minute at each level to a maximum of 3.5-3.7 bar and ii) Sunk Sand, pressure was kept at the maximum pressure for the entire haul.

Habitat Effects

A key environmental effect was the mechanical action of the dredge on the seabed; this was investigated by examining the excavation depth of dredge. At the Seal Sand site the speed of the vessel was 0.6 knots, with a starting pressure of 1.63bar (mean) this resulted in an excavation depth of about 86mm. As the pressure was increased to 2.47bar (mean) the excavation depth increased to about 192mm and increased again to about 240mm when the pressure was increased to 3.7-3.74bar (mean). At the Sunk Sand site the speed of the vessel was reduced to 0.3-0.4 knots and the pressure was increased to its working maximum of 3.57bar, during the haul this resulted in an average of 163mm of seabed excavated. The results also suggested that the excavation depth may be limited the composition of the sediment, in areas where the sediment appeared to be denser the excavation appeared to be reduced.

Species and Community Effects

Target species: there was a positive relationship between the speed at which the dredge was towed and the damage rate observed, the faster the speed of the vessel the greater the damage rate of Ensis directus that was observed. At faster speeds up to 35% of the catch were damaged this was considered to be unacceptably high.

Location

The Wash & North Norfolk Coast

Fishing Types

Hydraulic dredge. Suction dredge.

Reviewed by

Sewell et al 2007

Full Reference

Bell, C.M. & Walker, P., 2005. Desk study to assess the impact of cockle suction dredging on The Wash and North Norfolk Coast European Marine Sites. . No 670

Description

Review of evidence of the impacts of cockle dredging on the target species, their environment and non-target species. Including possible mitigation methods that can reduce the impact of suction dredging and the identification of gaps in current knowledge requiring future research.

Habitat Effects

As a result of cockle suction dredging various studies have indicated that the sediment has become unstable due to a loss of fine sediment and an increase in medium grain size. The direct removal of cockles and other bivalves has also led to sediment becoming unstable, since bivalve pseudofaeces play an important role in the binding of sediment.

Species and Community Effects

Suction dredging for cockles in the Wash is unlikely to impact on populations of knot, however may contribute to effects on oystercatcher populations, due to a link between oystercatcher mortality and the supply of cockles and mussels. A number of non-target species share the same community as cockles and are therefore likely to be affected by suction dredging, these include: polychaetes, the amphipod Bathyporeia sarsi, the snail Hydrobia ulvae and the bivalve Macoma balthica.

Looking at the changes in population numbers of oystercatchers and knots in the Wash and if these changes were linked to changes in shellfish stocks and winter weather. Bird counts have been carried out monthly since 1970; indices of population size were calculated using the Underhill method for December to February counts. Estimates of cockle and mussel stock and spat abundances were obtained from CEFAS and ESFJC (quantitative estimates available for mussels stocks from 1982 onwards and cockles from 1990 onwards).

Habitat Effects

Species and Community Effects

Oystercatcher – Population numbers increased rapidly in the 1980s reaching a peak of 40,000 in 1988/89, before crashing by 1998/99 the population of wintering birds was 25% of its population 10 years earlier.
Knot – Population numbers declined between 1970/71 dropping from 70,000 to 20,000, before recovering in the mid-1980s to 80,000. Numbers remained steady reaching a peak between 1990/1992 of 110,000 before dropping to 40,000 by 1998/99.
Cockle spat levels have shown declines since 1987, stock levels despite showing no trend since 1970 have shown declines in recent years. Spat indices showed poor recruitment for mussel larvae since 1987 and a rapid decline in stock abundances in 1982 (20,000t to 2,000-4,000t).
Results indicated that cockle or mussel spat abundance or winter weather had no effect on the survival rate of the knot. However, in poor cockle years and with declines in mussel stocks as a result of overfishing the vulnerability of oystercatchers to mass mortality was increased. Therefore to prevent further declines in the oystercatcher populations it is important that mussels are available in years when the cockle stocks are poor. The introduction of mussel cultivation is beneficial and an important tool for maintaining bird populations.

Location

The Wash, eastern England.

Fishing Types

Cockle fishery (mixed). Competition for resources (general).

Reviewed by

Sewell et al 2007

Full Reference

Cook, W, 1991. Studies on the effects of hydraulic dredging on cockle and other macroinvertebrate populations 1989-1990. .

Description

In 1989 a 3 month experimental study of the impacts of hydraulic suction dredging for cockles was carried out. As this was the first large scale use of the method the impact on both the cockles and other macroinvertebrate populations were considered. A second 3 month study was also conducted in 1990 again looking at both cockles and non-target species. Stock surveys were carried out before, during and after both studies. 5 transects were established with 55 sampling sites along each transect, at each site a 0.1m2 quadrat was laid and sediment within was dug out to a depth of about 7cm, the contents was sieved and retained for analysis.

Habitat Effects

The habitat type varied across the survey area. For the areas that appeared to have a muddy composition dredge tracks remained for several weeks after dredging had finished. However the sediment appeared mobile and quickly filled in the gaps back up to bed level. In areas where the bed substrate was harder, dredge blades were set so they would dig deeper into the sediment; this led to localized erosion of the bed surface. When dredging finished in October 1989 a number of hollows and hummocks could be seen across the seabed. However, by the end of January 1990 after are some strong winds the seabed had started to flatten out and return to a more normal appearance.

Paper looks at changes in oystercatcher numbers, survival, recruitment of juveniles to the winter population and body condition. Oystercatchers were caught using cannon nets between 1980 and 2003, where they were ringed, aged (divided into two age classes’ juveniles < 1 yr, adults > 1 yr), measured, weighed and released. The study period was divided into periods of high and low mussel stock; this was because cockle stock were unknown for most of the years due to the changing mussel stocks.

Habitat Effects

Species and Community Effects

The number of oystercatchers has fluctuated over the years, with declines in 1983/84 to a low in 1990/91, since than a recovery has been seen reaching a peak in 2001/02 after which there is no data available. This lowest count coincided with two consecutive years of suction dredging for cockles (1989 & 1990) and low mussel stocks. It seems that shellfishing and food supply does have an impact on birds, as a result of overfishing in the 1980s and 1990s the cockle and mussel stocks collapsed. During this period oystercatcher populations fell from 40,000 to 11,000 birds, as food supplies became low instead of moving elsewhere large numbers of birds died in three winters.

A review of site based information on these species, life history, distribution, habitat, reproductive biology and sources of threat. Together with recommendations to better assess and implement actions to help with the conservation of each species.

General review of ecology and conservation of three lamprey species. The review includes very brief explanations of threats to the species by exploitation.

Habitat Effects

Species and Community Effects

River lamprey (lampern) Young and adult lampreys are targeted by anglers fo fishing bait where they occur. Young larvae are dug, reducing populations and damaging their habitat. Adults are caught using traps and indiscriminate trapping could damage populations.

Sea LampreySimilar threats to those described for the river lamprey from anglers are described.

Species Action Plans The harbour porpoise Phocoena phocoena is vulnerable to incidental capture in fishing gear (unspecified). The pink seafan Eunicella verrucosa is sensitive to entanglement in fishing gears and resulting damage to soft tissues sometimes resulting in the death of colonies. The species may also be impacted by intensive potting and netting and direct collection as souvenirs. The fan shell Atrina fragilis is extremely vulnerable to mobile fishing methods. There is evidence that the bivalve has been wiped out in areas where scallop dredging takes place. Although they can survive some physical damage to the anterior end of the shell by mobile gears, they cannot survive removal from the sea bed. The native oyster Ostrea edulis has been severely impacted by the introduction of non-native species and diseases associated with bivalve mariculture. Over fishing has also severely impacted native oyster populations around the UK. The filamentous red algae Anotrichium barbatum , found on gravel and pebbles in Cardigan Bay may be vulnerable to bottom trawling. Detached knotted wrack, Ascophyllum nodosum is directly collected for alginates and collection has led to the 'decimation' of populations in the Uists.

Grouped Species Action Plans It is likely that commercial fisheries reduce the availability of prey species for piscivorous baleen whales, particularly the minke whale Balaenoptera acutorostrata and several species of toothed whale, although demonstrating such effects is extremely difficult. Entanglement in fishing gear (unspecified) is known for some species of baleen whale, but is not considered to be a significant problem in the UK.

In the approaches to the English Channel and the Celtic Sea, common and white sided dolphins are caught in substantial numbers in pelagic trawls. Between 1992 and 1993, 1200 striped and 500 common dolphins were caught by French drift-net fisheries between southern Ireland and the Azores. Between 1990 and 1995, post-mortem studies on 138 stranded common dolphins revealed that at least 62 percent had been killed as a result of bycatch.

Turtles are vulnerable to incidental entanglement in fishing gear and drowning. Damage to deep-water Lophelia reefs by various fisheries can reduce the habitat available and adversly affect a range of deep-sea fish species.

Habitat action plansSabelaria alveolata reefs can be damaged by trampling associated with fishing and collection of shore animals. Individual worms are also occasionally extracted and used as fishing bait.

Mudflats may be adversly affected by fishing activities and bait collection. Bycatch of juvenille flatfish in shrimp fisheries could be a problem as could bycatch associated with hydraulic dredging for shellfish.

Sheltered muddy gravels, found mostly in estuaries, inlets and bays are subjected to bivalve fisheries, which are currently small but may increase in the future. These habitats are also vulnerable to invasion by non-native slipper limpets associated with bivalve mariculture. Dredging for oysters and mussels, trawling for shrimp or fin fish, net fishing and potting can all cause physical damage to erect Sabellaria spinulosa reef communities and fisheries are thought to be the most important threat to this type of habitat. In the past, shrimp fishers have been known to actively seek out and fish over reefs for the pink shrimps Pandalus montagui. Fishing with mobile gears has been very destructive to horse mussel beds in the past, leading to the destruction of beds in Strangford Lough and of the coast of the Isle of Man. Trawls and dredges can 'flatten' clumps causing fatalities and a loss of associated fauna. Physical disturbance associated with trampling and fishing can be damaging to seagrass beds as can the effects of non native species introduced by bivalve mariculture. Mobile fishing gear, especially scallop dredges can devastate maerl beds by breaking and burying the thin layer of living maerl and have been particularly damaging in the Clyde Sea. Deepwater mud habitats (below 20 - 30 metres deep) are subject to potting and dredging for Nephrops. Mobile gears extract non-target organisms and disturb the seabed, whilst pots and creels are far less damaging. Marine fish farms sited above deep mud can affect the seabed by causing smothering and increased biological oxygen demand in mud. The tall sea pen Funiculina quadrangularis is susceptible to damage by mobile gears and is not found in Nephrops trawling grounds in the North Sea. Serpulid reefs are large and fragile and may be susceptible to damage by mobile fishing gears and anchors. They may also be damaged by direct impact from large pots or creels. Sublittoral sands and gravels are impacted by a wide range of fishing types. Some species occuring in these habitats (e.g scallops) are extracted directly by fisheries, others are removed as bycatch. Large, slow growing species are sensitive to fishing disturbance, whilst species inhabiting already perturbed seabeds are usually more resiliant. The removal of predators and competitors may effect the ecological functions within communities. Demersal trawls can break of larg pieces of Lophelia reef and repeated use of heavy 'rock-hopper' gear is known to flatten large areas of reef.

Review of marine turtle bycatch from UK waters, based primarily on data held in the database 'TURTLE', which contains 712 historic records of turtle sightings, strandings and bycatch from UK and Irish waters.

Habitat Effects

Species and Community Effects

Five species of marine turtle have been recorded from UK waters (all of these are listed in Annex IV of the Habitats Directive). Occurences of bycatch by various fishing types are discussed. Fixed gears, towed gears, driftnets ropes and lines associated with pot fisheries are all implicated as sources of turtle bycatch throughout the ranges of the turtles. Fishing may effects turtles from coastal waters to deep pelagic waters.

Study looks at the regeneration of native oyster stocks within the United Kingdom, taking into consideration the following: the biological factors, the technical requirements, the framework involved in regulating oyster farms and the economic consequences. The study also considers the current status and various attempts at restoration within the UK and from an international perspective, including Channel Islands, Ireland, France, Spain, Denmark, The Netherlands, Norway, New Zealand and the USA.

Rees, E.I.S 1996. Environmental effects of mechanised cockle fisheries: a review of research data The Ministry of Agriculture Fisheries and Food.. 42pp

Description

Review. Environmental effects fall into several broad categories the most obvious being (a) direct impacts, mainly on the benthic biotopes and on the discarded undersize by-catch (b) indirect interactions with predators and scavengers, including shorebirds, (c) ancillary disturbance from the vessels and vehicles, including effects at the shore access points.

Habitat Effects

Hydraulic dredge tracks can be seen at low tide days or weeks later, persistence depending on the stability of the sediment surface and the prevailing tide or wave conditions. On areas of cohesive sediment the tracks appeared to act as lines from which erosion of the surface layer spread out therefore appearing to accelerate the erosion phase of a natural cycle of cohesion of the surface sediment by worm tube mats. Where dredging has been carried out in a sheltered area with eel grass (Auchencairn Bay) breaking the sward allowed erosion that produced clearly visible grooves down the shore. Long-term effects on benthic diatoms on and in the surface of intertidal flats were considered unlikely.

Species and Community Effects

Shell breakage occurs with overall damage rates to cockles and Macoma baltica in screen rejects from hydraulic dredgers 12.6 percent and 5.3 percent respectively. In experimental plots where damage rates from tractor dredging were determined these were 9.3 percent in an area of muddy sand and 8.2 percent in a sandy area but only impinged directly on about 80-85 percent of the area of the plots. Dredged areas often had a lot more dead shell scattered on the surface, an effect which can persist for several months whereas in undisturbed beds most dead shell is normally under the surface which can create a shell layer limiting the depth to which small drainage channels can normally erode into a cockle flat.

Observation on other species include the tendency for some motile species, like the amphipod Bathyporeia sarsi to temporarily leave disturbed areas, lugworms producing normal casts in dredge tracks as soon as the tide falls, tubes of the sand mason worm L. conchilega still standing, apparently to nearly their full extent in the hydraulic dredge tracks. Results from a study of tractor dredging in the Burry Inlet recorded declines in other invertebrates (particularly H. ulvae, P. elegans and N. hombergii), the greatest fall being 14 days after dredging for the less mobile species in the muddy areas, and increases in some species Urothoe sp., M. balthica, A. tenuis. Localised additional bird activity has also been reported in some areas following dredging. In a study on the Solway Firth it was concluded that because natural changes are very large the fishery may not have a significant effect on bird numbers unless a high proportion of the cockles are harvested. On sandy areas the effect on most invertebrate populations was considered to be causing some thinning of stocks rather than persistent patchy defaunation. In muddier, more cohesive sediments tracts may persist for months. Persistent hydraulic dredging has in some cases been reported to have changed the sediment structure which may have medium term consequences for deposit feeding benthic species. The most undesirable effects are where the surface is bound by swards of eel-grasses.

Study compared the changes in sediment characteristics and the abundance of three bivalve species in the Dutch Wadden Sea as a result of mechanical cockle dredging. Sediment samples were taken before the cockle fishery took place (Sept. 1988) and again in 1992, core samples for benthos and sediment were taken at 500m intervals along transects around Griend. The activities of the fishing vessels during the study period were recorded along with stock size and settlement densities.

Habitat Effects

From 1988 to 1994 silt was lost from the sediments near the Griend that were dredged for cockles and the median sediment grain size increased. By 1996 the initial characteristics of the sediment were re-attained.

Species and Community Effects

The abundance of Macoma balthica declined for 8yrs after the removal of all Mytilus edulis and most of the Cerastoderma edule. From 1989 to 1998 the stock levels of Macoma balthica, Mya arenaria and Cerastoderma edule declined and did not recover to the levels in 1988. The declines were not helped by the fact that there were low rates of settlement until 1996 in the fished areas. A comparison of settlement both in the short and medium term in several areas that had be dredged for cockles and areas that hadn’t been dredged showed that dredging had a significant negative effect on subsequent settlement of Cerastoderma edule, (Macoma balthica declined but not significantly). The loss of both Cerastoderma edule and Macoma balthica was the most evident in areas that were dredged for cockles. The study concluded that suction dredging for Cerastoderma edule had long-lasting negative effects on the recruitment of bivalves, especially the target species.

Location

Wadden Sea

Fishing Types

Mechanical cockle dredge.

Reviewed by

Sewell et al 2007

Full Reference

Diederich, S., 2006. High survival and growth rates of introduced Pacific oysters may cause restrictions on habitat use by native mussels in the Wadden Sea Journal of Experimental Marine Biology and Ecology, 328, 211-227

Description

Study examining potential loss of settlement habitat for Mytilus edulis due to invasion by Crassostrea gigas on mudflats.

Habitat Effects

Species and Community Effects

The authors conclude that through competition for space and altering substrate composition, that invasions by C. gigas would be likely to restrict usable habitat for native mussels in the Wadden sea area. Oysters were not selective with regard to settlement substrate, whilst mussels showed significantly higher growth on clear sand flats.
High survival rates of oysters will compensate for poor recruitment years, resulting in the successful spread of the species.

Investigation into the potential impact of a policy of immediately discarding all by-catch from shrimp fisheries in the North Frisian Wadden Sea.

Habitat Effects

Species and Community Effects

Clearance rate of discards estimated by feeding crabs and shrimps in aquaria. Traps baited with discards used to examine fate in sublittoral and take by birds assessed using combination of counts, photography and video recording. Underwater video revealed grey seals feeding on discarded fish.

Authors conclude that 1988 seabird population in the area would have easily been capable of clearing the discards of moribund roundfish. Harbour seals which were most likely to benefit from flatfish discards.

Review of ecology and distribution of both species of shad and both species of anadromous lamprey found in Welsh coastal waters. The report includes descriptions of threats to the species and this includes some reference to threats from fishing activities.

Habitat Effects

Species and Community Effects

Twaite shad and allis shad Probably the most important fishing related mortalities are from fish traps, particularly putcher net fishermen targeting salmon near Lydney. Fixed net shrimp fishermen also catch occasional fish. Shad are regularly reported in trawl catches and due to the high levels of trawling in some regions, this may cause considerable losses. These species may also be caught as bycatch during bass trawling. The authors note that herring stocks around Wales are improving and should a fishery for this species recommence, it may pose a significant threat to this species.

River lamprey The most important fishing related mortality of this species are caused by fish traps in estuarine waters. Accidental capture by trawling in marine waters appears to be rare. The species is likely to escape through the mesh of commercial nets due to their size and shape.

Sea lampreyThe most important fishing related mortality of this species are caused by fish traps in estuarine waters were they concentrate during upstream spawning migrations. Catches in trawls are rare. Reduction in numbers from historical levels may be due to reduced abundance of the favoured prey species including the salmon, sea trout and possibly the shad from coastal waters.

Surveys were undertaken of three sites before and after trawling. Site One: Dulas Bay. Sediment was coarse sand with gravel and low commercial fishing activity. Site Two: Red Wharf Bay. Sediment was medium sand, occasionally fished. Site Three: Walney Island. Muddy sediment, heavily fished. 4 m wide beam trawl was used at all sites. Eight 0.75 m wide Newhaven, scallop dredges were used at Site Two only.

Habitat Effects

Species and Community Effects

At Site One, numbers of hermit crab Pagurus bernhardus increased following trawls. At Site Two, no increase in scavangers was observed immediately after trawling, however 25 hours after fishing, the abundance of some scavanging starfish and brittle stars increased significantly. At Site Three, the abundance of some previously abundant scavanging species decreased following trawling disturbance. Damage to large, fragile organisms was observed by divers, following trawls.

Paper looks at the breeding performance of two gulls and aims to determine what factor may be influencing the differences that have been observed in the populations. Ratios of birds feeding at different sites and their diets were considered as were any disease incidents.

Habitat Effects

Species and Community Effects

Results indicated that the number of breeding pairs of the herring gull has decreased from about 17,000 to 4,000 between 1969 and 2005, where as the number of breeding pairs for the lesser black-backed gull has remained between about 15,000 and 24,000. A number of factors are likely to be involved in the differences between the two populations and the availability of food is likely to be one. In Morecambe Bay the number of mussel fishermen has increased over the past 10 years and recently cockle fishing has taken place, as a result the more intertidal feeding herring gull populations may have been negatively affected during both breeding and wintering periods by reduced shellfish availability. A decrease in landings suggests that the amount of food available around the docks for foraging birds has also declined affecting particularly the herring gull. However, the fishery discards at sea, which favour the lesser black-backed gull, do not appear to have changed.

Study examining the effects of dredging for scallops on megafauna by direct observations of damage in bycatch and in dredge tracks (individuals encountering dredges, but not captured). Authors used two gangs of four 'Newhaven' spring toothed dredges. An identical damage score was used by divers, surveying dredge tracks and scientists on board vessels examining bycatch. The abundance and damage score was recorded for all megafauna.

Habitat Effects

Species and Community Effects

Asterias rubens and Neptunea antiqua were more severely damaged in bycatch than dredge tracks. Cancer pagurus was more severely damaged in the dredge track. For Cancer pagurus and Liocarcinus spp. nearly twice as many crushed or damaged animals were left on the sea bed than were found in bycatch. Some species were little affected by dredging, including Porania pulvillus and Asterias rubens. The study showed that the majority of fauna to come into contact with the dredge remains on the seafloor and that the majority of megafauna mortality associated with scallop dredges of this type occurs in dredge tracks and not in discarded bycatch.

The study examined deep-water coral related bycatch associated with commercial catches taken during a trawl along the continental shelf break, west of Ireland. Some large coral concretions that were taken in trawls were carbon dated. This was combined with observations of trawling impacts on a Lophelia pertusa reef in the shelf waters off western Norway.

Habitat Effects

Video footage: There was an obvious difference between trawled areas of Lophelia pertusa reef and untrawled areas. Trawled areas were severelly damaged and characterised by coral rubble and sparse living, broken and dislodged colonies of coral. Untrawled sites had far higher habitat complexity and more sessile, filter-feeding macrofauna including sponges.

Species and Community Effects

Bycatch study: Large amounts of coral debris and associated coral species were brought up in five out of 229 observed trawls. Coral fragments had a wide diversity of associated epifauna and were up to 1 m 2 in diameter. Based on fragments of the coral species Desmophyllum cristagalli found in trawls was estimated to be 4550 years old.

Paper looks at the operating procedures of the wild harvest pearl aquaculture system in Western Australia and the potential environmental impacts from the industry. Data from two separate studies were examined and a risk analysis workshop was held with the aim of documenting the main potential ecological and environmental risks that arise from various activities carried out by the industry. Thirteen risks were identified across the industry and entered into a risk matrix of likelihood and consequence. Scores from 1 to 6 (1 = remote, 6 = very likely) were assigned and multiplying to two gave a position on the matrix, a score more than 20 was considered to have a high concern, a score of 7-19 was considered to have a moderate concern and a score below 6 was considered to have a low concern.

Habitat Effects

Species and Community Effects

13 different risks were identified, however none were considered to have a high concern and 3 risks produced a moderate concern: 1) introduction of disease from seedling, 2) attraction of other fauna and 3) introduction of exotic organisms. The remaining 10 risks were considered to be of low concern: 1) spread of disease, 2) introduction of disease form the hatchery, 3) introduction of disease from translocation, 4) impact to protected and endangered species resulting from entanglement, 5) reduction of primary productivity, 6) potential for litter, 7) perceived change in water quality, 8) nutrient impacts in sediment, 9) impact to protected and endangered species attracted to farm lighting and 10) impact of habitat. The workshop concluded that the environmental impacts of the Pinctada maxima industry in Western Australia were small.

The paper aimed to asses the factors controlling the distribution and abundance of the sea scallop Placopecten magellanicus and evaluate the spatial relationship amongst megafaunal invertebrates that also inhabit the ground at two sites. However, coincidentally one of the sites (Fippennies Ledge) was dredged prior to the second year of data collection, which provided a chance to observe the effect that scallop dredging had on the megafaunal community. Five dives were made in a manned submersible in 1986 (2 at Jeffreys Ledge and 3 at Fippennies Ledge), in 1987 the Fippennies Ledge was revisited and 6 transects were completed. For each dive analysis four data sets were available: i) 35-mm photographs, ii) videotapes, iii) transcript of the driver-scientists’ observations and iv) the ships bridge log.

Habitat Effects

The most obvious difference between the dredged site in 1987 and the pre-dredged site in 1986 was a change in substratum from a more organic-silty sand to a sandy gravelly appearance. This was said to be as a result of disruption to the tube mats produced by the amphipod Erichthonius sp., which were abundantly found in box core samples. Since no diving took place at a later date, it was no possible to determine how long the effects would have lasted.

Species and Community Effects

Three species of megafauna were dominating at both sites, the burrowing certianthid anemone Cerianthus borealis, the sabellid worm Myxicola infundibulum and the sea scallop Placopecten magellanicus, all three of the species showed a large scale cluster distribution. As a result of the marked increase in scallop dredging between 1986 and 1987 at Fippennies Ledge there was a marked decrease in the density of all three megafaunal species, Placopecten magellanicus declined by 70% , and both Cerianthus borealis and Myxicola infundibulum declined by 25-27% .

Describes the results of two separate surveys. The first is a study of the short-, medium- and long-term effects of otter trawling for the Norwegian lobster in the Irish Sea over inshore and offshore muddy sediments (See Ball et al. 2000 [a or b] for full details of survey and results). The second is a study in a sheltered Scottish sea loch. The loch had been closed to fishing for 25 years and experimental trawls were undertaken monthly for 16 months, followed by an 18 month, monitored recovery period.

Habitat Effects

Gareloch study: Trawl marks on the sea bed and changes to sea bed roughness were identifyable for five months after trawling but were almost indistinguishable, but still visible after 18 months.

Species and Community Effects

Gareloch study: The number of species and individuals increased throughout the trawling disturbance period, although biomass did not alter significantly. At the same time measure of species diversity and evenness decreased in trawled areas. This change was attributed to an increase in the number of opportunistic species.Large bivalves and polychaetes were identified as being sensitive to trawling disturbance. Community changes occurred between treatment and control sites and were still apparent 18 months after trawling.

Location

Western Irish Sea and Loch Gareloch, Scotland.

Fishing Types

Otter trawl (demersal).

Reviewed by

Sewell & Hiscock 2005

Full Reference

Fisheries Research Services 1998. A Study of the effects of water jet dredging for razor clams and a stock survey of the target species in some Western Isles populations Marine Laboratory, Aberdeen. Report No. 8/98

Description

Experimental dredging in sandy areas swept by strong tidal flow with a paucity of epifauna but openings of numerous larger infaunal animals such as various bivalve species. Tests conducted using single fishing events rather than repeat fishing.

Habitat Effects

Trenches up to 2m wide and 0.15 deep at centre were observed. These started to fill after 5 days and were no-longer visible after 11 weeks but sediment in the tracks remained fluidised under a thin crust of firm sediment. Long term physical effects are less well understood and may be exacerbated by repeated fishing of the same area.

Species and Community Effects

Immediate reduction in number of species, individuals and biomass in fished tracks but measures of diversity showed no effects. Abundance of polychaetes reduce and of amphipods increase. Crab species moved into the region to scavenge of material disturbed by the dredge.The results suggest biological effects are only short term. No effects were recorded after 11 weeks. Species likely to be damaged (eg.heart urchins and large bivalves) were rare in the samples but present in dredge catches where damage was noted.

Most of the animals in the sediments are adapted to a mobile environment so other than being removed or displaced they were not thought to be greatly affected by the dredging. On the basis of this work difficult to comment on areas with more obvious and diverse epifauna. Authors conclude there is little difference between the biological impact of water jet dredges and suction dredging although the latter may have a greater physical effect and fish less selectively.

Survey of intertidal benthic community and physical characteristics at a site of commercial clam cultivation on a shallow shelving mudflat during clam growth and post harvesting. Underlying sediment composed of London clay interspersed with shell debris and lignin deposits. Surface sediment of fine silt and sand with patches of clay.

During clam growth no significant increase in faunal diversity under clam lay but density of benthic species individuals much greater. Community under clam lay significantly different from the control areas. Control area dominated by polychaete Nephtys hombergii, area under clam lay dominated by deposit feeding worms Lanice concilega and the bivalve Mysella bidentata. Nets may change hydrography reducing water flow and increasing sedimentation. This increases food supply and so may promote larval settlement. Adjacent areas may be influenced by commercial clam operation.

Suction dredge harvesting had a profound effect on the community structure. Large amounts of sediment and associated animal community (particularly crustaceans and bivalves) removed. Seven months post harvesting density of individuals decreased significantly to the point where there was no difference between control and harvested sites, with Neptys hombergii responsible for the similarity between treatment and control. The effect of clam harvesting barely detectable after 7 months. Clam cultivation increases productivity as the netting reduces wave action and other disturbances.

Eel grass beds: induced oyster reefs can directly reduce growth of zostera marina. Crasosstrea beds are unlikely to encourage the same biodiversity levels as Zostera beds, but both encourage higher levels than nearby clear sand or mud.
Rocky shores: Oyster growth may increase habitat heterogeneity on clear rock surfaces and may provide more surface area for the settlement of barnacles.
In Argentina, Shore birds have been shown to spend a disproportionate amount of time and higher foraging rates in areas of higher C.gigas settlement.

Species and Community Effects

Most serious effects include introduction of ‘hitchhiking’ species. Where non-native species become established, they may out compete with native species. Crassostrea gigas is able to filter feed at a greater rate that Ostrea edulis and grows more quickly, giving it a competitive advantage.
The report also suggests areas for further study.

Study comparing the effects of raking and digging for clams in eelgrass beds over one season. The author examined the effect of mimicked, small scale, recreational digging and raking for clams on eelgrass beds and their associated macro and megafauna.

Habitat Effects

Species and Community Effects

In raked treatments, some loss of plant biomass was noted imediately after raking, but no differences were found between treatment and control plots after two weeks indicating that eelgrass beds recovered quickly following this type of disturbance. In contrast, sites where digging had taken place were slower to recover and differences between control and treatment plots were still evident 10 months after disturbance. No significant difference between macrofauna or megafauna was found between treatment and control plots for both raking and digging sites.

The paper examines the mortality of both Redshanks and Oystercatchers that were found dead after a severe winter (for Redshanks the data could be compared to that of normal samples taken in October and November 1978). The collected carcasses were weighed and measured and then deep frozen till analysis. In order to measure the fat and protein reserves the carcasses were dried in a vacuum oven and the fat was extracted using petroleum ether in a Soxhlet apparatus. Fat was expressed as a lipid index and protein reserves were measured as indices of pectoral muscle size.

Habitat Effects

Species and Community Effects

The results indicated that level of fat and muscle in the Redshanks found dead both in 1979 and 1982 showed extensive use when compared with those in a normal condition, indicating that the birds died in severe whether after using almost all of their fat and protein stores. The small amount that was left is likely to be structural and not available as a reserve. The results from the Oystercatchers founds dead indicated the same situation as they had very small fat and muscle reserves (were able to compared with one bird found in a normal condition). Samples from the Ythan estuary (windy and moderately cold weather, Jan/Feb 1979) were compared with samples from the Montorse Basin (prolonged cold, but calm weather, Jan 1982) to determine if the mortality of Redshanks and Oystercatchers was caused by an inability to mobilize reserves as opposed to resulting from the exhaustion of body reserves. The results indicated that the fat reserves could be mobilized fast enough, however once the fat reserves had been exhausted then the energy requirements had to be supplied by the breakdown of proteins. The cause of death in 1982 was likely to be due to an inability to breakdown the proteins fast enough to balance the rate of heat loss, rather than the exhaustion of protein followed by starvation.

Location

Ythan Estuary (Grampia) and Montrose Basin (Tayside)

Fishing Types

Disturbance (general). Competition for resources (general).

Reviewed by

Sewell et al 2007

Full Reference

Jefferies, D.J., 1989. Further records of fyke net and creel deaths in British otters Lutra lutra with a discussion on the use of guards. Journal of the Otter Trust, , 13-19

Description

Further reports of otter deaths in fyke nets and creels. These include 2 males in fyke nets in the upper Ythan estuary after nets in the river for only 3 days, indicating the speed at which an eel fyke net will operate as an otter trap in a catchment with normally high otter density. Also reports the release of an otter from a fyke net providing an example of otter surviving capture when in shallow water if struggles bring the cod-end to the surface.

Habitat Effects

Species and Community Effects

Data confirm the potential of eel fyke to attract and kill otters living at very low density. Also appears to be considerable attraction when silver eels begin their seasonal migration - August/September on East Coast, October/November in Severn. This must be one of the last opportunities for otters to feed on eels in quantity before capture becomes too difficult until spring. Overall monthly distribution of all drownings in fykes, creels and fish traps shows a marked concentration in autumn and winter. Partly explained by seasonality of fishing but also when main food may be reduced for seasonal reasons.